DEPARTMENT OF THE AIR FORCE OPERATING ROOM VENTILATION UPDATE

HUMAN SYSTEMS CENTER (AFMC)

BROOKS AIR FORCE BASE, TEXAS

22 Jul 98

MEMORANDUM FOR ALL MEDICAL FACILITY/SG

FROM: Det 1 HSC/OEMI

2402 E Drive

Brooks AFB TX 7823

SUBJECT: Consultative Letter, AL-OE-BR-CL-1998-0096, Medical Facility Ventilation Requirements for Infection and Contaminant Control

1. This letter provides guidance on recommended ventilation requirements in medical facilities. Attachment 1 outlines ventilation requirements for infection control. Attachment 2 discusses control of chemical contaminants. The information provided is compiled from guidance of the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), the Centers for Disease Control and Prevention (CDC), and the National Institute for Occupational Safety and Health (NIOSH). We have included the information that we think most of you will need. For ventilation requirements in areas not listed in the attachment, refer to the 1995 ASHRAE Applications Handbook for more complete guidance. The guidelines provided are for fixed medical facilities; additional guidance will be forthcoming for mobile hospitals, such as Air Transportable Hospitals (ATHs).

 

"See The National Resource Centers publications #29 and #31."

2. You should be aware that the specific requirements for your medical facility may vary from those in this letter, depending on the agency providing oversight to your facility, and local and state health agencies. We recommend you contact these agencies to see if they have more stringent requirements than provided in this letter. The Joint Commission on Accreditation of Healthcare Organizations (JCAHO) does not set specific ventilation requirements, but looks at infection rates to identify locations where ventilation systems may be inadequate. For requirements on new construction, refer to MIL-HDBK-1191, DoD Medical Military Construction Program Facilities, Design and Construction Criteria.

3. This letter has been coordinated with HQ AFMOA, the USAF/SG Chief Consultant for Infection Control, and the Air Force Medical Services Agency. Please distribute this letter to the members of your Infection Control Committee. Your local expert on ventilation is Bioenvironmental Engineering. If you have questions about ventilation requirements for your specific needs, or comments on this letter, please contact the Industrial Hygiene Branch at DSN 240-6137 or ihbranch@guardian.brooks.af.mil.

//signed//

GARY N. CARLTON, Maj, USAF, BSC

Chief, Industrial Hygiene Branch

2 Atch

1. Ventilation Requirements for Infection Control

2. Control of Chemical Contaminants

cc: ALL MAJCOM/SG/SGPB

ALL MEDICAL FACILITY/SGPB

AFMSA/SGSF (Maj Steele)

59 MDW/CMI (Maj Difato)

AFMOA/SGOE (Capt Sobel)

Attachment 1: Medical Facility Ventilation Requirements for Infection Control

1. Background: Heating, Ventilation, and Air Conditioning (HVAC) systems in hospitals and clinics serve several purposes. The most important related to infection control are: (1) controlling infectious agents released into the air during medical procedures or carried by infected patients; (2) maintaining temperature and relative humidity levels conducive to patient therapy and comfort, and (3) inhibiting bacterial growth and virus activation. Examples of infectious agents of concern in a medical facility include Myobacterium tuberculosis and Legionella pneumophila (tuberculosis and Legionnaire’s disease), highly infectious bacteria that can be transported by HVAC systems. Varicella and Rubella (chicken pox/shingles and German measles) are viruses that are virulent within air. Some molds such as Aspergillis can be fatal to immunocompromised patients. Bacteria generally can be removed by filtration because they are typically present in colony-forming units larger than 1 mm. Airborne viruses that transmit infection are submicron in size; however, there is no current method to filter 100% of the viable particles. Therefore, ventilation-pressure relationships are the primary means to prevent the airborne transmission of infectious agents.

 

"See The National Resource Centers publications #29 and #31."

 

2. Ventilation Basics

a. Infection problems frequently are traced to bacterial or viral sources inside the facility. Outdoor air is relatively free of bacteria and viruses (assuming there aren’t contamination sources in the vicinity of outdoor air intakes); therefore, outdoor fresh air will dilute bacterial and viral contamination. Maintaining proper pressure relationships between functional areas through a properly designed HVAC system can facilitate removal of airborne infectious agents in the hospital or clinic.

b. There are two general types of HVAC systems for dilution and removal of contaminated air: the single-pass system (100% outside air) and the recirculating system. In the 100% outside air system, conditioned outside air passes through the room or area, and 100% of that air is exhausted to the outside. In a recirculating system, a portion of the exhaust air is replaced with fresh outside air, which mixes with the portion of exhaust air not discharged to the outside. The resulting mixture (that can contain a large percentage of contaminated air) is recirculated into either the facility’s general ventilation or a specific room or area.

c. Supply air diffusers deliver air to the area and provide a desired distribution pattern. They may be round, square, rectangular, linear slots, louvered, fixed, adjustable, or a combination. Return air grilles are louvered or perforated coverings for HVAC openings and exhaust air from the area through the return air system. They may be located in the sidewall, ceiling, or floor. These grilles may be directly connected to an open return air plenum (a space above the ceiling) or to a ducted return air system.

d. Air should be supplied and exhausted so a continuous flow of air through the area will occur. Poor placement of supply diffusers and exhaust grilles result in localized recirculation or short-circuiting (passage of air directly from the air supply to the air exhaust), reducing the amount of fresh air reaching the occupied zone. The air supply and exhaust should be located so clean air flows across infectious sources and toward the exhaust.

e. In general, the HVAC system should provide air movement from clean to less clean areas. Areas that require air free of contaminants, such as operating rooms, should be positively pressurized relative to adjoining rooms or corridors to prevent air flow into the clean area. A positive pressure differential is

maintained by supplying more air to the area than is exhausted from it. Contaminated areas, such as infectious isolation and autopsy rooms, should maintain a negative air pressure relative to adjoining rooms and corridors.

"See The National Resource Centers publications #29 and #31."

 

A negative pressure differential is maintained by supplying less air to the area than is exhausted. An equal pressure occurs when the amounts of air supplied and exhausted from the area are the same. Differential air pressures are maintained only in closed rooms. It is important to ensure the doors and windows have a good fit between pressurized areas. If a room is poorly sealed, you may want to consider installing weather stripping and drop bottoms on doors to prevent reductions in pressure differentials between areas.

f. Differential pressure-sensing devices can be used to monitor room pressures, either periodically or continuously. The minimum pressure difference resulting in air flow into a room is very small, 0.001 inch of water (0.00004 pounds per square inch [psi]; atmospheric pressure is 14.7 psi). Because of this small pressure differential, a very sensitive pressure-sensing device is required to ensure accurate measurements. Pressure-sensing devices should monitor the room pressure just inside the air flow path into and out of the room, such as the bottom of the door. Poorly distributed air can result in pressure variations in the room; if the pressure-sensing port cannot be located directly across the air flow path, you will need to confirm that the pressure at the sensing port is the same as the pressure across the air path.

3. Ventilation System Evaluation

a. Evaluate the mixing effectiveness in the room or area of interest. Release smoke from smoke tubes at a number of locations in the room and observe the smoke movement. Movement of smoke in all parts of the room is an indicator of good air mixing. If the air in some parts of the room stagnates, indicating poor mixing, relocation of the supply diffusers and exhaust grilles may be necessary.

b. Locate individual supply air diffusers and return air grilles. Use smoke tubes or a piece of paper to determine which diffusers/grilles supply air and which ones exhaust air. Measure both the supply and exhaust ventilation volumetric flows in the area of interest. The preferred method is to measure volumetric flows in cubic feet per minute directly, such as with an Alnor^® Balometer. If you don’t have a Balometer, or the diffuser/grille is too large to use a Balometer, you will have to use the face velocity method. Measure average air velocity out of the diffuser or into the grille with either a swinging vane or thermal anemometer, and multiply by the diffuser/grille manufacturer’s calibration factor to obtain volumetric flow. If you don’t have the manufacturer’s calibration factor (very likely), multiply the average air velocity by the diffuser/grille surface area and a correction factor accounting for the type of diffuser/grille in use. See the ACGIH Industrial Ventilation: A Manual of Recommended Practice for appropriate correction factors. Make sure doors and windows to the area are closed to prevent reduction of the pressure differential during measurements.

c. Calculate air exchange rates in the area of interest.

Use the supply air volume for areas requiring equal or positive pressure, and the exhaust air volume for areas requiring negative pressure. Be careful to identify all supply and exhaust locations in the area; sometimes adjacent areas need to be included in the supply and exhaust volumes. For example, most

isolation rooms have bathrooms that are continuously exhausted. Failing to include the bathroom in the total exhaust volume for the isolation room will result in underestimation of the actual exhaust air volume.

d. To confirm the pressure characteristics in the location you’re evaluating, we recommend you also perform smoke tube tests. Blow smoke at the bottom of the room door, approximately 2 inches in front of it, or at the face of a grille or other opening if the door has such a feature. If there is no opening and the bottom of the door is covered, slightly crack open the door. If the room requires a negative pressure, stand outside the door and visibly check to ensure air is being drawn into the room. If the room requires a positive pressure, stand inside the room and ensure air is being drawn out of the room. Blow the smoke slowly from the tube to ensure the velocity of the smoke does not overpower the air velocity. Of course, make sure nobody is on the other side so they don’t get a whiff of irritant smoke. An alternative is to install a differential pressure measuring device in the room.

e. In addition to taking ventilation measurements, identify the locations of your outdoor air intakes. Your facility management personnel should be able to determine their locations. Check to ensure that there are no obvious sources of contamination near the intakes, such as garbage or infectious material storage areas. Also ensure there are no sources of engine exhaust that could be drawn into the building, such as vehicles and emergency diesel generators. The intakes should be located at least 30 feet from ventilation exhaust outlets, cooling towers, surgical vacuum systems exhaust, and plumbing vent stacks.

4. Specific Criteria: Table 1 summarizes ventilation requirements in areas that directly affect patient care in existing facilities. The required air balance is indicated by the following symbols:

++ supply flow exceeds exhaust flow by 20%

+ supply flow exceeds exhaust flow by 10%

even supply and exhaust flows are equal

- exhaust flow exceeds supply flow by 10%

Ventilation rates are in room air changes per hour (ACH). Air recirculation must not be used in areas marked "No" due to difficulty in cleaning the air and the potential for build-up of contamination. For rooms with local exhaust ventilation installed, additional air will be needed to maintain proper pressure relationships.

a. Surgery/Critical Care: Surgery suites are the most critical areas in a medical facility requiring aseptic conditions. The best air delivery system has supply diffusers located on the ceiling over the operating table, with exhaust grilles located on opposite walls near the floor. This arrangement results in the movement of clean air through the breathing zone of the surgical team toward the contaminated floor area, the most effective air movement pattern to maintain acceptable contaminant concentrations. Above-ceiling areas should not be used as a return air plenum in these areas.

b. Infectious Isolation: Infectious isolation rooms are used to protect the medical staff from the patients’ contagious diseases, so the room must be under negative pressure relative to adjoining areas. ASHRAE recommends a minimum of 6 ACH for infectious isolation rooms, based on comfort and odor control considerations. How effective this air flow is in reducing the concentrations of airborne pathogens is not well characterized, so CDC recommends the air flow be increased to greater than 12 ACH to reduce the concentration of droplet nuclei in tuberculosis isolation rooms. If the facility HVAC system cannot achieve this ventilation rate, auxiliary room-air recirculation methods should be used (fixed room-air HEPA recirculation system, wall- or ceiling-mounted room-air HEPA recirculation system, or portable room-air HEPA recirculation unit). Ultraviolet germicidal irradiation may be used to supplement any of the ventilation methods for air cleaning. Air from isolation rooms should be exhausted directly to the outside of the facility and away from supply air intakes and populated areas. If recirculation of air from these rooms is unavoidable, the air should be HEPA-filtered before recirculation. For more details on infectious isolation rooms, see Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Facilities, CDC Morbidity and Mortality Weekly Report, 28 October 1994 (http://www.cdc.gov/epo/mmwr/mmwr.html).

c. Protective Isolation: Protective isolation rooms are used to protect immunosuppressed patients who are highly susceptible to diseases, so these rooms must be under positive pressure. In cases where the patient is both immunosuppressed and contagious, the isolation room should be balanced to provide an equal pressure with respect to adjacent areas.

"See The National Resource Centers publications #29, #94 and #31."

d. Central Sterile Supply: There is no standard terminology for Central Sterile Supply, so different agencies may use the same terminology to refer to different areas. For the purposes of this guidance document, Central Sterile Supply generally has three functional areas: a decontamination area where dirty instruments are cleaned, washed and dried, then packaged in plastic bags or wrapped in linen cloths; a sterilizer equipment area containing ethylene oxide gas and steam sterilizers; and a sterile storage room where sterilized instruments are aerated and kept. In some medical facilities, the functional areas may be separated into more than one room, such as the decontamination area being separated into an instrument decontamination room and a preparation/packaging room.

 

"See The National Resource Centers publications #29, #31,#94 and #96."

If the functional areas are all contained in the same room, air should flow from the sterile storage and sterilizer equipment areas toward the dirty decontamination area.

e. Ancillary Services: In laboratories that have local exhaust hoods installed, supplementary air will be needed to provide hood make-up air and keep proper pressure relationships. Hoods where infectious materials are used require ultrahigh efficiency filters on the exhaust outlet. The hospital should have procedures in-place to remove and replace the contaminated filters. The exhaust air from laboratory hoods should be discharged to the outdoors with no recirculation. The autopsy room is subject to heavy bacteriological contamination; exhaust grilles should be located at both the ceiling and low on the walls. Discharge exhaust air from the autopsy room above the roof of the facility.

5. Air Filtration: Fresh outdoor air requires filtration to remove dust and dirt. The filtration efficiencies provided here are the minimum efficiencies the medical facility should have; JCAHO may require higher efficiencies based on the specific procedures being performed in your facility. Air provided to surgery/ critical care areas and isolation rooms should be filtered through a minimum of two filter beds. Filter bed #1 should be located upstream of the HVAC equipment and should have a filter efficiency of 25%. Filter bed #2 should be downstream of the supply fan, recirculating spray water systems, and water-reservoir humidifiers, and have a filter efficiency of 90%. For orthopedic and transplant operating rooms, cardiothoracic surgery, heart lung pump rooms, and neurosurgery, a third HEPA filter (99.97% efficient) is recommended. The air provided to Central Sterile Supply and laboratories should be filtered through at least an 80% efficient filter.

6. Non-Compliance with Recommendations: Most Air Force medical facilities were built under older design criteria than listed in Table 1. For example, ventilation criteria for operating rooms for a long time were 15 ACH for recirculating HVAC systems. Therefore, there is a high likelihood that your HVAC systems will not meet Table 1 criteria. So what do you do next? Obviously, you’re not going to shut-down the operating room because you have 15 ACH instead of 25 ACH. Use the following to prioritize necessary actions.

a. Pressure Relationships: The most important criteria is to ensure the area of interest has the proper pressure relationship to adjacent areas. For example, if the operating room is under negative pressure, there is a high potential for contamination leaking into the room during surgical procedures. Isolation rooms with improper pressure relationships will not provide the necessary patient isolation. Improper pressure relationships in Central Sterile Supply can result in contamination of surgical instruments. These are problems that require immediate correction, either through balancing of the HVAC system or system redesign.

b. Exhaust Air Discharge: The next priority is ensuring contaminated air exhausted from rooms is not redistributed into clean areas. For example, if the air exhausted from the isolation or autopsy rooms is not all exhausted to the outside, contamination could end-up in other areas of the medical facility. If this situation is due to improper balancing of the HVAC system, it should be corrected immediately. If it’s due to a design flaw in the HVAC system, the medical facility should program to correct the flaw. In the interim until the situation is corrected, air being exhausted from the rooms should be filtered to remove the contamination.

c. Air Flow Patterns: Ensuring optimal air flow patterns in rooms and preventing stagnation and short-circuiting of air is the next priority. If the supply diffuser and exhaust grille locations do not permit complete mixing of the supply air into all parts of the room, the "effective" ACH in the room may be less than required for adequate control of infectious agents. Moving supply and exhaust openings may be a less expensive alternative than increasing room ventilation rates and re-balancing the facility’s HVAC system. Increasing ventilation rates in a room with poorly distributed air will not be very effective in most cases.

d. Ventilation Rates: Although important, the actual ventilation rate is not as critical as the pressure relationship, the location of the exhaust air discharge, and room air flow patterns. For example, if the HVAC system for an operating room using recirculated air has 15 ACH total air instead of the recommended 25 ACH, but is under positive pressure, has a good distribution of supply air, and the medical facility has no indications of infection problems during surgeries, then there should be no great urgency to upgrade the system to 25 ACH. However, if infection control records indicate infection problems, the medical facility should look at a near-term upgrade to the system. All medical facilities should program for long-term upgrades of their HVAC systems to the criteria in Table 1.

7. Frequency of Measurements: Even though there are no regulatory requirements mandating periodic testing of the HVAC system, routine checks of the system are essential to ensure proper pressure relationships are maintained. Medical facility HVAC systems are fairly complex, consisting of outdoor intakes, dampers, air cleaning units, heating/cooling coils, fans, humidifiers and dehumidifiers, and supply and exhaust ducts. Poor or infrequent maintenance can render the HVAC system ineffective in controlling infectious agents. Damper opening settings can change (either on their own volition or through purposeful adjustment). Cleaning filters and ducting can become clogged. Air flow into and out of supply diffusers and exhaust grilles can be reduced by placing medical equipment or furniture near the vents. These changes can imbalance the HVAC system, change pressure relationships, and reduce volumetric air flows. Therefore, we recommend that you measure room air change rates semiannually. Isolation and surgical rooms should be checked at least monthly for proper pressure relationships by smoke tubes, even if pressure-sensing devices are installed. CDC recommends isolation rooms be checked daily for negative pressure while being used for infectious isolation.

Table 1. Pressure Relationships and Ventilation Requirements in Medical Facilities¹

 

Location	Pressure Relationship	Outdoor Air (ACH)	Total Air (ACH)	Exhaust All Air to Outside?	Air Recirculated Within Room?	Temperature (° F)	Relative Humidity (%)
Surgery/Critical Care
Delivery room
All outdoor air	++	15	15	Optional	No	68-76	50-60
Recirculating air	++	5	25	Optional	No	68-76	50-60
Trauma room2	+	5	12	Optional	No	68-76	50-60
Intensive care unit	+	2	6	Optional	No	75-80	30-60
Nursery suite	++	5	12	Optional	No	75	30-60
Operating room
All outdoor air	++	15	15	Yes	No	68-76	50-60
Recirculating air	++	5	25	Optional	No	68-76	50-60
Patient room	even	2	4	Optional	Optional	75	30-503
Recovery room	even	2	6	Optional	No	75	50-60
Isolation
Infectious isolation	-	2	12	Yes	No	75	30-503
Protective isolation	+	2	15	Yes	HEPA4	75	30-503
Central Sterile Supply
Decontamination	-	2	6	Yes	No	Comfort	Comfort
Sterilizer equipment	-	Optional	10	Yes	No	Comfort	Comfort
Sterile storage	+	2	4	Optional	Optional	Comfort	Comfort
Ancillary Services
Autopsy rooms	-	2	12	Yes	No	Comfort	Comfort
Laboratories	-	2	6	Yes	No	Comfort	Comfort
Pharmacy	+	2	4	Optional	Optional	Comfort	Comfort
Radiology
X-Ray (surgery)	+	3	15	Optional	No	75-80	40-50
X-Ray (diagnostic)	even	2	6	Optional	Optional	75-80	40-50
Darkroom	-	2	10	Yes5	No	Comfort	Comfort

 

1.Table adapted form ASHRAE HVAC Applications Handbook (1995)

2.Treat operating room within the emergency room as operating room

3.30% in winter; 50% in summer and exposures below occupational exposure limits

^{4.Recirculation allowed if supply air is HEPA filtered}

^{5.Optional if film processor has local exhaust ventilation installed}

Attachment 2: Control of Chemical Contaminants in Medical Facilities

1. Background: Besides controlling infectious agents, the medical facility’s HVAC system can also help control hazardous chemical contamination and odors. For example, general room ventilation in an operating room, the primary purpose of which is infection control, will reduce concentrations of anesthetic gases released during surgical procedures. However, in some specific procedures and processes general room ventilation may not be adequate to reduce contaminant levels below applicable occupational exposure limits. In these cases, local exhaust ventilation may be necessary. This attachment identifies some chemical contaminants of interest in medical facilities, their health effects, sampling methods, and control recommendations.

2. Anesthetic Gases

a. Source of Exposure: Among the anesthetic gases commonly used in operating rooms are nitrous oxide (N₂O), halothane (C₂HBrClF₃), enflurane (C₃H₂OClF₅), isoflurane (C₃H₂OClF₅), and desflurane (C₃H₂OF₆). The principal source of waste anesthetic gases in operating rooms is leakage from equipment, particularly when the anesthetic gas is administered through a face mask. This leakage can occur in the following ways: gas may escape during hook-up and check-out of the system; excess gas may seep over the lip of the patient’s mask; the patient may exhale gas into the room; leaks may occur in the anesthetic breathing system; or scavenging systems may be used improperly. A related problem is exposure of recovery room personnel to waste gas exhaled by post-operative patients.

 

"See The National Resource Centers publications #29 and #31."

 

b. Toxicology: All anesthetic gases are, by their very nature, central nervous system depressants, and exposure results in decreased mental performance, audiovisual ability, and manual dexterity. However, they are of special concern because of worries over adverse reproductive effects, specifically spontaneous abortions and congenital abnormalities in offspring. Risks of hepatic and renal diseases are also increased among exposed workers. The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) for N₂O and halothane are 50 parts per million (ppm) as an 8-hr time weighted average (TWA). The 8-hr TLV-TWA for enflurane is 75 ppm. The other anesthetic gases do not have established TLVs. The Occupational Safety and Health Administration (OSHA) has not set Permissible Exposure Limits (PELs) for any of these chemicals.

c. Sampling Methodology:

(1) Nitrous Oxide: Sampling for waste N₂O gas can be tricky. NIOSH Method 6600 requires collection of breathing zone air in a sample bag through an air sampling pump, and subsequent analysis by a portable infrared spectrophotometer (e.g., Foxboro^® MIRAN). Having operating room personnel wear a sample bag during surgical procedures can be a problem, because the bag can interfere with the procedure. Therefore, using a calibrated MIRAN to take direct readings in the worker’s breathing zone may be your best option. If you perform real-time sampling, attach the sampling train to the lapel of the worker on the side closest to the patient; concentrations in this location best represent worker exposures. If you don’t have a MIRAN or using it in your operating room is impractical, consider OSHA Method 166. This method uses passive N₂O dosimeters. Several companies make these dosimeters, such as Landauer (marketed under the trade name NITROX^®), but the analysis can be costly. In addition to personal sampling, room area sampling is recommended. These readings won’t give you personal exposure measurements, but can be useful in determining how effective the operating room’s HVAC and scavenging systems are in controlling waste gases.

(2) Organic Anesthetics: There is no established NIOSH sampling method for the organic anesthetics; your best option is to use one of the OSHA Methods. OSHA Method 103 (halothane, enflurane, isoflurane) uses either Anasorb^® CMS or Anasorb^® 747 sorbent tubes. OSHA Method 106 (desflurane) uses Anasorb^® 747 sorbent tubes. Anasorb is a trade name for a type of sorbent tube marketed by SKC^®. Anasorb CMS is a carbon molecular sieve prepared from Saran-type precursors. Anasorb 747 is a beaded synthetic carbon of low ash content. If wearing sampling pumps in the operating room is a problem, then consider using passive organic vapor monitors to take screening samples.

d. Controls:

(1) Anesthetic Delivery System: Exposures to waste anesthetic gases result from leaks in the anesthetic delivery system. Leaks can occur from the high-pressure connections (gas storage tanks, wall connectors, hoses connected to the anesthetic machine, and the anesthetic machine itself), or low-pressure connections (anesthetic flowmeter and scavenging mask). The leakage results from loose-fitting connections, loosely assembled or deformed slip joints and threaded connections, and defective or worn seals, gaskets, breathing bags, and hoses. In addition, rubber and plastic components can be degraded by the anesthetic gases. All high- and low-pressure connections, tubing, and breathing bags should be regularly checked for damage and leaks. Use a soap bubble solution for the high-pressure connections and a calibrated MIRAN for the low-pressure connectors. All connectors should be periodically replaced.

(2) Scavenging System: Also known as a waste anesthetic gas exhaust (WAGE) disposal system, a scavenging nasal mask consists of a shroud large enough to capture anesthetic gases exhausted from the patient’s mouth. An inner mask is contained within a slightly larger outer mask. A slight vacuum present in the space between the masks scavenges gases exhaled by the patient, as well as excess gas from the anesthesia machine that leaks around the edges of the inner and outer masks. Hoses lead to a vacuum pump that removes the scavenged anesthetic gas. Leakage can be more difficult to control in dental operatories than in general operating rooms because only the patient’s nose is covered during administration and scavenging. There should be an automatic interlock system ensuring the anesthetics cannot be turned on unless the scavenging system is also on. Most scavenging systems come with a calibrated flowmeter, permanently connected to the scavenging system vacuum hose, with an acceptable exhaust flow range indicated by the manufacturer. If the recommended exhaust flow is not known, the scavenging system should have a minimum exhaust flow of 45 liters per minute (lpm) to minimize leakage. The scavenging pump should be capable of maintaining the required scavenging flow at each mask if more than one mask is in use at a time. Scavenged gas should be vented to the outside of the facility away from fresh air intakes, windows, and walkways. Avoid venting scavenged gas into a recirculating ventilation system.

"See The National Resource Centers publications #29, #31,#94 & #96."

 

(3) General Room Ventilation: The primary purpose of room ventilation in operating rooms is to control infectious agents, but ventilation will also help control gases that leak during anesthesia administration. If area concentrations of N₂O in the operating room are above 25 ppm (action level), you may want to consider increasing air flow into the room or the amount of outside fresh air to allow for further dilution of anesthetic gases. This is especially critical if the facility does not have a separate vacuum system to remove scavenged gas, and the waste gas is exhausted through the existing HVAC system. 100% outdoor air should be provided to the operating room if waste gas is exhausted through the HVAC system. If concentrations of anesthetic gases remain elevated, consider installation of supplementary local exhaust ventilation (hood or duct) to capture leakage at the source.

3. Ethylene Oxide

a. Source of Exposure: Ethylene oxide (C₂H₄O, EtO) is a highly flammable and reactant gas used in Central Sterile Supply as a sterilizing agent for medical equipment and supplies. EtO is usually supplied in either compressed gas cylinders (containing 88% Freon and 12% EtO) or single-dose cartridges of 100% EtO. Items to be sterilized are wrapped and sealed with an EtO exposure indicator tape and loaded in the sterilization chamber. The door is closed and the sterilizer is put through a cycle, the specific parameters depending on the items being sterilized. The chamber is charged with EtO and heated. At the end of the sterilization cycle, some sterilizers go through an aeration cycle, while others require immediate unloading. The sterilizer door is typically cracked open a few inches for approximately 15 minutes before the sterilized items are removed from the chamber. The load is then removed and placed in an aeration chamber to collect EtO that can off-gas from the wrapping material.

b. Toxicology: EtO is an eye and respiratory irritant, causes central nervous system depression, and has been linked to leukemia and spontaneous abortions. OSHA considers it a potential occupational carcinogen (29 CFR 1910.1047). The OSHA PEL and ACGIH TLV is 1 ppm as an 8-hr TWA. OSHA also has set a 15-min short-term exposure limit (STEL) of 5 ppm.

c. Sampling Methodology: The preferred sampling method is NIOSH Method 1614, which uses a solid sorbent tube (HBr-coated petroleum charcoal). An alternate method is OSHA Method 49, which uses a 3M Ethylene Oxide passive dosimeter. There is one other method, NIOSH Method 3702, but it requires collection of a bulk air sample and analysis by a portable gas chromatograph, so it probably won’t be one most bases will have the capability to do.

d. Controls: Sterilizers should be located in separate unoccupied rooms that have a negative pressure in relation to adjacent areas and at least 10 ACH general room ventilation. A dedicated local exhaust system should be installed to remove the sterilizing gas from the sterilizer; refer to the manufacturer’s literature for required volumetric flows. A slot or canopy hood should be placed above the sterilizer door to remove EtO that vents from the sterilizer when the door is cracked open. See the ACGIH Industrial Ventilation: A Manual of Recommended Practice for design details. Contaminated air exhausted by both the HVAC and the local exhaust systems should not be recirculated; this will prevent re-entry of the gas into other areas of the facility. Some jurisdictions require installation of equipment to remove EtO from exhaust air. Ensure sterilized materials and their packaging are aerated in aeration cabinets; studies show that approximately 5% of the EtO in the sterilizer remains in these items. Metal and glass materials that do not absorb EtO do not require aeration unless they are wrapped. For more information on controlling EtO in medical facilities, refer to NIOSH Publication #89-120, Control Technology for Ethylene Oxide Sterilizations in Hospitals.

 

"See The National Resource Centers publications #29, #31 & #96."

 

4. Formaldehyde

a. Source of Exposure: Formaldehyde (CH₂O) is a gas but is usually used as formalin, a 37-50% solution by weight of formaldehyde gas. It is used as a disinfectant, sample preservative, and embalming fluid. You can expect to find formaldehyde exposures in histopathology, autopsy, and the medical laboratory.

b. Toxicology: Formaldehyde is an irritant, causes sensitization dermatitis, and is linked to nasal cancer. It is classified as a suspected human carcinogen. The OSHA PEL is 0.75 ppm as an 8-hour TWA and 2 ppm as a 15-min STEL (29 CFR 1910.1048), while the ACGIH TLV is 0.3 ppm as a ceiling limit. Here’s a short history lesson. The PEL and TLV were both 1 ppm as an 8-hr TWA at one time. The PEL was amended in 1992 to 0.75 ppm mainly over concern for the cancer risk of formaldehyde. The TLV was revised in 1989 to 0.3 ppm as a ceiling limit, primarily to reduce sensory irritation. Since Air Force policy (AFOSH Std 48-8) states to use the most stringent value, you should use the TLV as the occupational exposure limit. Of course, if you comply with the ACGIH TLV ceiling value, you also will be in compliance with the OSHA PEL.

c. Sampling Methodology: There are many ways to sample for formaldehyde. Which one you choose depends on the what information you’re interested in. The most sensitive method is NIOSH Method 3500, and is best for comparison to the 0.3 ppm ceiling limit. However, it requires a 1-mm teflon membrane filter in-line with two impingers in series containing 1% sodium bisulfite solution, so it’s not the easiest method to use. NIOSH Method 2541 uses a solid sorbent tube (2-hydroxymethyl piperdine treated XAD-2), so it’s easier to use, but has a higher limit of detection than Method 3500. There are also several companies that market passive formaldehyde samplers, but you should view them useful only for screening sampling. You should use one of the NIOSH methods if you’re determining compliance with exposure limits. These passive samplers are also relatively expensive and you’ll have to pay for the analysis, which can be costly. MSA markets a continuous formaldehyde monitor under the trade name "Formaldemeter." It has a sensitivity of 0.2 ppm, less than the ACGIH TLV of 0.3 ppm as a ceiling level. Although we cannot vouch for its reliability, if you calibrate the monitor properly, it could help you determine if the ACGIH TLV is exceeded. Other companies also make direct-reading instruments that can be used as personal or area monitors.

d. Controls: If large quantities of formaldehyde are used in the autopsy room, local exhaust ventilation may be necessary to keep concentrations below occupational limits. Studies have shown that the best design is to place exhaust take-offs on each side of the autopsy table. Contaminated air is drawn through the exhaust slots and away from the pathologist. A slot velocity of 2000 feet per minute (fpm) and a total exhaust volume of 725 cubic feet per minute (cfm) is recommended. Histopathology generally has a specimen table or lab hood containing some sort of local exhaust ventilation. If formaldehyde levels in histopathology are elevated even with the use of exhaust hoods, consider the use of ventilated glove boxes, if practical. These units completely encapsulate the operation, allowing the pathologist hand access to the tissue samples through two entrance ports. These glove boxes have viewing shields so the work area is visible during examination.

5. Glutaraldehyde

a. Sources of Exposure: Glutaraldehyde (C₅H₈O₂) is a broad-spectrum anti-microbial cold sterilant/disinfectant for hospital equipment. It is commonly used to disinfect endoscopes, bronchoscopes, and other hospital instruments. Glutaraldehyde is generally used as a 2% solution (marketed under the brand name CIDEX), but is available in solutions up to 50%.

b. Toxicology: Glutaraldehyde is an eye and respiratory irritant and is associated with dermatitis and skin sensitization. The current ACGIH TLV is 0.05 ppm as a ceiling limit. OSHA has not established a PEL for glutaraldehyde.

c. Sampling Methodology: You have two sampling methods to choose from: the NIOSH Method 2532, which uses a solid sorbent tube (silica gel coated with 2,4-dinitrophenylhydrazine), and OSHA Method 64, which uses glass fiber filters treated with 2,4-dinitrophenylhydrazine and phosphoric acid. The NIOSH 2532 is preferred over the OSHA 64, because the NIOSH 2532 sample media is less expensive and does not require refrigeration. In addition, MSA also makes a direct-reading instrument called the "Glutaraldemeter" that has a sensitivity of 0.03 ppm. As with their formaldehyde meter, we cannot vouch for its accuracy and reliability. There are several other companies marketing direct-reading instruments for glutaraldehyde.

d. Controls: Rooms where glutaraldehyde is used should have a minimum of 10 ACH general room ventilation. If sampling indicates exposure limits are exceeded, then consider installation of a laboratory hood to control vapors. Gluteraldehyde soaking bins should be placed inside the hoods to limit vapor evolution into occupied areas. Several companies market "ductless" glutaraldehyde hoods that are available in a variety of sizes depending on the application, but we cannot vouch for their effectiveness. You may want to note that OSHA has fined hospitals for wearing latex rubber gloves when handling glutaraldehyde; the chemical can cause the gloves to deteriorate, allowing permeation. Use disposable nitrile rubber gloves with glutaraldehyde.

"See The National Resource Centers publications #29,#94, #96 and #31."