HETA 95–0031–2601

 University of Medicine and Dentistry of New Jersey,

University Hospital 

Newark, New Jersey

Teresa A. Seitz, MPH, CIH

John Decker, CIH

Paul Jensen, PhD, PE, CIH

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports
ii

PREFACE

The Hazard Evaluations and Technical Assistance Branch of NIOSH conducts field investigations of possible

health hazards in the workplace. These investigations are conducted under the authority of Section 20(a)(6)

of the Occupational Safety and Health Act of 1970, 29 U.S.C. 669(a)(6) which authorizes the Secretary of

Health and Human Services, following a written request from any employer or authorized representative of

employees, to determine whether any substance normally found in the place of employment has potentially

toxic effects in such concentrations as used or found.

The Hazard Evaluations and Technical Assistance Branch also provides, upon request, technical and

consultative assistance to Federal, State, and local agencies; labor; industry; and other groups or individuals

to control occupational health hazards and to prevent related trauma and disease. Mention of company names

or products does not constitute endorsement by the National Institute for Occupational Safety and Health.

ACKNOWLEDGMENTS AND AVAILABILITY OF REPORT

This report was prepared by Teresa A. Seitz and John Decker of the Hazard Evaluations and Technical

Assistance Branch, Division of Surveillance, Hazard Evaluations and Field Studies (DSHEFS). Desktop

publishing was performed by Ellen Blythe.

Copies of this report have been sent to employee and management representatives at UMDNJ–University

Hospital and the OSHA Regional Office. This report is not copyrighted and may be freely reproduced.

Single copies of this report will be available for a period of three years from the date of this report. To

expedite your request, include a self–addressed mailing label along with your written request to:

NIOSH Publications Office

4676 Columbia Parkway

Cincinnati, Ohio 45226

800–356–4674

After this time, copies may be purchased from the National Technical Information Service (NTIS) at

5825 Port Royal Road, Springfield, Virginia 22161. Information regarding the NTIS stock number may be

obtained from the NIOSH Publications Office at the Cincinnati address.

For the purpose of informing affected employees, copies of this report shall be

posted by the employer in a prominent place accessible to the employees for a

period of 30 calendar days.
iii

Health Hazard Evaluation Report 95–0031–2601

University of Medicine and Dentistry of New Jersey

University Hospital 

Newark, New Jersey

October 1996

Teresa A. Seitz, MPH, CIH

John Decker, CIH

 Paul Jensen, PhD, PE, CIH

SUMMARY

In July 1995, the National Institute for Occupational Safety and Health (NIOSH) conducted a health hazard

evaluation (HHE) at University Hospital of the University of Medicine and Dentistry of New Jersey. The HHE

was conducted in response to a management request for assistance in evaluating a supplemental high efficiency

particulate air (HEPA) filtration system installed in one of University Hospital’s new ambulances. The purpose

of the HEPA filtration system is to reduce the potential for tuberculosis (TB) transmission during transport of

patients with active TB by filtering the vehicle’s air through a HEPA filter before returning it to the ambulance.

HEPA filters are capable of removing at least 99.97% of particles > 0.3 microns (:m) in diameter (the most

penetrating particle size), thus they are believed to be highly efficient in removing Mycobacterium

tuberculosis–containing droplet nuclei which are approximately 1–5 :m in size.

NIOSH investigators conducted a field evaluation of particle clearance in an ambulance equipped with a

supplemental HEPA filtration system. A similar ambulance without the HEPA filtration system was also evaluated

for comparison. A polystyrene aerosol tracer was nebulized in the ambulances, and the decay in particle counts

was measured at four locations using laser particle counters. Using this data, effective air change rates in the

ambulance were calculated under varying conditions of operation. 

The results indicated that the ambulance with the supplemental HEPA filtration system cleared particles

faster than the ambulance without the supplemental system, when tested under similar conditions. The

effective air change rates ranged from about 43 to 46 air changes per hour (ACH) in the HEPA–equipped

ambulance, and from 26 to 32 ACH in the non–HEPA–equipped ambulance. These comparisons were

made using two different ambulances (the HEPA ambulance was a 1995 model and the non–HEPA

ambulance was a 1994 model, same manufacturer). Undefined operational or design factors could have

biased the effective ACH rates for both ambulances. This evaluation also showed that particle clearance

could be improved by the use of the rear vent fan (when positioned on the “high” setting) in conjunction

with the provision of outside air through the vehicle’s main HVAC systems. For vehicles that do not have

supplemental HEPA filtration systems, using the vent fan and providing outside air through the vehicle’s

main HVAC system improves aerosol clearance and thus reduces the potential risk of TB transmission.

A recommendation concerning the use of respiratory protection by workers during transport of known or

suspected infectious TB patients is included in the report along with a recommendation for the

performance of regular preventive maintenance and leak testing of the HEPA unit.

Keywords: SIC 4119 (Ambulance service, road), tuberculosis, TB, ambulance, emergency medical services, EMS,

health care, HEPA filtration. 
TABLE OF CONTENTS

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Acknowledgments and Availability of Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Introduction ......................................................................... 2

Background ......................................................................... 2

Methods ........................................................................... 2

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Discussion and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Page 2 Health Hazard Evaluation Report No. 95–0031

INTRODUCTION

In October 1994, a management representative at

University Hospital/University of Medicine and

Dentistry of New Jersey (UMDNJ) requested

assistance from the National Institute for

Occupational Safety and Health (NIOSH) in

evaluating a high efficiency particulate air (HEPA)

filtration system in an ambulance, and in the

utilization of two portable HEPA filtration units in

the hospital. These engineering controls were being

considered for use as supplemental methods to

reduce the risk of tuberculosis (TB) transmission.

NIOSH investigators conducted a site visit in July

1995 to evaluate the HEPA filtration units.

However, the portable HEPA filtration units in the

hospital were not in use at the time of the site visit,

and the units were not evaluated. This report

summarizes the findings and recommendations from

the NIOSH evaluation of the HEPA filtration system

in the ambulance.

BACKGROUND

Because of the potential for exposure to airborne

Mycobacterium tuberculosis from patients with

known or unrecognized active tuberculosis, a

recirculating HEPA filtration system was installed in

a new ambulance to reduce the risk of airborne TB

transmission during patient transport. Seven

hospitals in the Newark area are served by University

Hospital’s ambulance service. Employees reported

that approximately 80% of the ambulance runs are

for non–emergency transport, and that many of these

patients are in high–risk categories for active

tuberculosis.

METHODS

On July 26–27, 1995, NIOSH investigators

conducted a site visit at University Hospital. To

evaluate the HEPA filtration system in the

ambulance, NIOSH investigators released a

polystyrene aerosol tracer and measured its decay

under varying field conditions. This allowed

assessment of the ability of the system to clear

particles from the air and to determine the effective

air change rate in the ambulance. To evaluate the

positioning and integrity of the HEPA filter, the

aerosol was also released at the exhaust grille, and

the particulate concentration was measured at the

return air diffuser. Monodisperse polystyrene

spheres (Duke Scientific, Palo Alto, CA, lot 13172)

were diluted with deionized, filtered water and

aerosolized in a Collison 6–jet nebulizer (BGI Inc.,

Waltham, MA). The aerosol was generated at a

flowrate of 20 liters per minute. The resulting

monodisperse aerosol had an aerodynamic diameter

of 3.004 microns (:m) and standard deviation of

0.029 :m.

The conditions used during successive runs included:

engine status (on or off), heating, ventilating and

air–conditioning (HVAC) system status

(recirculation mode or outside air provision), vent

fan status (on or off), and ambulance status

(stationary or mobile). Two emergency transport

vehicles were included in the field study. The first

was a 1995 Type III ambulance (unit 113) that had

been in service less than one month, and the second

was a 1994 Type III ambulance (unit 109) that had

been in service for over one year. Both were

manufactured by Excellance of Allentown, PA.

Only the 1995 model was equipped with the

supplemental HEPA filtration system. The HEPA

filtration system in the 1995 ambulance could not be

turned off while the ambulance was running.

Unfortunately, a comparable 1995 model without a

HEPA filtration system was not available. Instead,

we measured aerosol clearance for comparison

purposes in a 1994 model which reportedly was

nearly identical to the 1995 ambulance with respect

to its layout and HVAC system design. Both had

separate HVAC units for the cab and rear

compartments and a vent fan in the rear of the

ambulance capable of exhausting air directly to the

outside. A bulkhead door separates the front and rear

compartments, but is typically open allowing free

access between the cab and rear sections of the

ambulance. 
Health Hazard Evaluation Report No. 95–0031 Page 3

The VehiculaireTM recirculating HEPA filtration

system (Biological Controls, Eatontown, NJ) in

ambulance 113 operates independently from the

vehicle’s HVAC system. The HEPA unit contains a

pre–filter followed by a HEPA filter. The air intake

is at floor level directly behind the area where a

patient’s head would be positioned on the gurney.

Supply air is delivered to the area behind the middle

to rear portion of the gurney (see Figure 1). The

volume of supply and return air was measured with

a model 8370 AccuBalance® airflow–measuring

hood (TSI Inc., St. Paul, MN). A model 8360

Velocicalc Plus air velocity meter (TSI Inc., St. Paul,

MN) was used to determine the velocity of air

exhausted by the vent fan in the rear of the

ambulance. The airflow was calculated by

multiplying the air velocity by the area of the vent.

As shown in Figure 1, the aerosol was released in the

rear of the ambulance, on the driver’s side. To

improve initial mixing, a small circulation fan was

used during the aerosol release period. After a few

minutes, the nebulizer was turned off and particle

counts were continuously recorded using four Met

One model 227B laser particle counters (Met One

Inc., Grants Pass, OR). Three or four persons were

present in the ambulance during the aerosol

clearance evaluation. Efforts were made to restrict

movement within the ambulance to minimize the

disruption of airflow patterns. The particle counters

were positioned on the attendant seat, family

member seat, gurney, and on the background driver’s

seat. The particle counters were operated in the

automatic mode with a sample time of 10 seconds

and a hold time of one second. When the particle

counts returned to baseline, the run was ended. The

logarithmic rate of decline in particle counts 3.0 :m

and greater (natural log of particle count versus

elapsed time) was plotted and the least squares

methoda

 used to calculate a straight line that best fit

the data. In the resulting equation which describes

the line (y = mx + b), “m” represents the slope of the

line and is equal to the air change rate. The decline

in particle counts may be due to removal

mechanisms other than the vehicle’s HEPA filtration

system. These unquantified mechanisms probably

include dilution with outside air leaking into the

ambulance, impaction of particles on surfaces, and

removal by the vehicle’s main ventilation system. 

RESULTS

Figure 2 shows a typical plot of particle counts vs.

time for ambulance 113. The plot begins when the

aerosol release period has ended. When the engine

is turned on and the HEPA filtration system and

HVAC systems are operating, particle clearance is

accelerated. Note the change in the slopes of the

lines. Effective air change rates increased

dramatically from 3.5 to 39.1 air changes per hour

(ACH) when the ambulance ventilation system and

HEPA filtration system were activated. The data in

this example are from a particle counter positioned

on a gurney inside a stationary ambulance (unit 113).

Air change rates for runs conducted under varying

field conditions are shown in Table 1. As shown in

the table, some particle clearance occurs when the

engine is not running; however, this clearance

equates to a mean air change rate for the four

sampling locations of only 2.9 ACH. This clearance

may be due to settling/impaction of the aerosol on

surfaces inside the ambulance and infiltration of

outside air. For the HEPA ambulance, air change

rates were determined with the ambulance stationary

and mobile. There was little difference in air change

rates when all other conditions were held constant

(HEPA sys = on; vent fan = off; cab and rear A/C =

recirc), as evidenced by mean ACHs of 43.0 and 46.6

during stationary runs, and 45.0 and 46.4 during

mobile runs. While there were differences in air

change rates at the four sampling locations within a

given run, there were no apparent trends in terms of

some locations being consistently higher or lower

than others.

The mean air change rate was considerably higher in

the HEPA ambulance when the HVAC units were

 a The least squares method is a statistical

method of estimation that minimizes the sum of

the squared differences between the observed

values and the values predicted by the model.
Page 4 Health Hazard Evaluation Report No. 95–0031

allowed to bring in outside air and the rear vent fan

was positioned on “high” (ACH = 55.9, as compared

with 45.0 and 46.4, when recirculation mode was

used and the vent fan was off). Air flow

measurements indicated that the vent fan was

exhausting 91 cubic feet of air per minute (CFM)

directly to the outside when positioned on “high.”

For the non–HEPA ambulance, air change rates were

lower than in the HEPA ambulance when all other

conditions were the same (ambulance = mobile; cab

and rear HVAC = recirc; vent fan = off). The mean

ACHs for three separate runs were 25.8, 27.6 and

32.0 for the non–HEPA ambulance, as compared to

45.0 and 46.4 for two runs with the HEPA

ambulance. Unlike with the HEPA ambulance, the

mean ACHs were not appreciably higher when the

HVAC units were positioned to bring in outside air

and the vent fan was turned on. However, this may

be due to the fact that the vent fan was inadvertently

placed on the “medium” setting rather than the

“high” setting as had been done for the HEPA

ambulance.

The HEPA filtration system supply and exhaust air

flow rates ranged from 105 to 110 CFM. Ambulance

status (stationary or mobile) did not affect the

measured air flow rates. The positioning and

integrity of the HEPA filter was checked by releasing

the polystyrene aerosol at the unit’s exhaust (intake)

grille and measuring particle counts at the face of the

supply diffuser. This brief evaluation revealed that

the filter was effective in removing the polystyrene

aerosol as evidenced by very low or non–detectable

values for particles 3.0 :m or greater in size. Leak

testing of the HEPA filter was not performed.

Modifications to the system may be necessary to

allow such testing to be done.

DISCUSSION AND

CONCLUSIONS

NIOSH investigators conducted a field evaluation to

assess the performance of two Type III ambulances

in clearing particles from the air. One ambulance

had been retrofitted with a supplemental HEPA

filtration system to enhance particle clearance and

thus reduce the risk of airborne TB transmission. A

tracer aerosol with an aerodynamic diameter of

approximately 3 :m was used for the evaluation.

This aerosol is in the size range of Mycobacterium

tuberculosis (Mtb)– containing droplet nuclei (1–5

:m), and is also in the range where HEPA filters

remove nearly 100% of particles. The NIOSH data

showed that the ambulance with the supplemental

HEPA filtration system cleared particles faster than

the ambulance without the supplemental system,

when tested under similar conditions. The effective

air change rates ranged from about 43 to 46 ACH in

the HEPA ambulance, and from 26 to 32 ACH in the

non–HEPA ambulance. It should be noted, however,

that these comparisons were made using two

different ambulances (the HEPA ambulance was a

1995 model and the non–HEPA ambulance was a

1994 model, same manufacturer). Although we were

informed that the two ambulances were nearly

identical with respect to layout and HVAC system

design, there may be other factors that we did not

evaluate that could have influenced ACH rates in the

non–HEPA ambulance. Despite this fact, the

calculated air change rates for the non–HEPA

ambulance generally fell in the range specified in the

Federal Specification for the Star–of–Life

Ambulance.1

 This specification requires that the

ventilation system(s) of the driver and patient

compartments provide a complete change of ambient

air at least every two minutes (i.e., 30 ACH) with the

vehicle stationary. Although measurements in the

non–HEPA ambulance were made while the vehicle

was mobile, the air change rates measured in the

HEPA ambulance showed little variation between

stationary and mobile runs. 

This evaluation also showed that particle clearance

could be improved by the use of the rear vent fan

(when positioned on “high”) in conjunction with the

provision of outside air through the main HVAC

systems. For the HEPA ambulance, an effective air

change rate of about 56 ACH was calculated for the

run where the vent fan was used and outside air

provided, as compared with 45 and 46 ACH without

these additions. Thus for vehicles that do not have
Health Hazard Evaluation Report No. 95–0031 Page 5

1. GSA [1994]. Federal Specification for the

Star–of–Life Ambulance, KKK–A–1822D.

Automotive Commodity Center, Federal Supply

Service, General Services Administration.

November 1994, p 51–52.

2. OSHA [1993]. Enforcement policy and

procedures for occupational exposure to

tuberculosis. Washington, DC: Occupational

Safety and Health Administration. Memorandum

(10/20/93). 

3. CDC [1994]. Guidelines for preventing the

transmission of Mycobacterium tuberculosis in

health–care facilities, 1994. MMWR 1994;

43(No. RR–13).

4. Code of Federal Regulations. Respiratory

protection. 29 CFR 1910.134. Washington, DC:

U.S. Government Printing Office, Federal

Register.

supplemental HEPA filtration systems, using the

vent fan and providing outside air through the main

vehicle HVAC system will improve aerosol

clearance and potentially reduce the risk of TB

transmission. Opening windows may also be

beneficial in diluting potential contaminants such as

Mtb, but may not always be appropriate for other

reasons (noise, temperature and humidity control,

etc.).

RECOMMENDATIONS

1. While supplemental engineering controls such as

the HEPA filtration unit described above can

improve particle clearance in the ambulance, they

will not eliminate the potential for exposure to

droplet nuclei containing Mtb. Thus, respiratory

protection should be used by EMS personnel during

transport of patients who have, or are suspected of

having active tuberculosis. The Occupational Safety

and Health Administration’s (OSHA) enforcement

procedures for tuberculosis2

 require that respirators

meeting the criteria outlined in the most recent CDC

guidelines3

 be used by workers when transporting

such patients in enclosed vehicles. The OSHA

guidelines require at a minimum the use of

NIOSH–approved N95 respirators. These respirators

should be used in conjunction with a respiratory

protection program meeting the requirements

outlined in the OSHA respirator standard.4

2. The manufacturer of the HEPA filtration system

should be consulted to determine the best method of

leak testing the HEPA filter, and the desired

frequency of such testing. Leak testing is an

important part of a preventive maintenance program

for HEPA filters and is particularly important in this

situation because of the potential for considerable

movement and vibration during ambulance runs. In

addition, periodic visual inspections should be made

(e.g. weekly) to check the positioning and evaluate

the integrity of the pre–filter and HEPA filter. 

REFERENCES
Page 6 Health Hazard Evaluation Report No. 95–0031

TABLE 1

AIR CHANGE RATES BASED ON AEROSOL CLEARANCE IN TYPE III AMBULANCES WITH AND WITHOUT

SUPPLEMENTAL HEPAFILTRATION

HETA95–0031

UNIVERSITY HOSPITAL

NEWARK, NEW JERSEY

EFFECTIVE AIR CHANGES PER HOUR (ACH)†

CONDITIONS

RUN

DATE 

ATTENDA

NT 

SEAT

FAMILY 

MEMBER

SEAT

BACK–

GROUN

D

DRIVER 

GURNE

Y

MEAN

ACH

HEPA AMBULANCE #113

ENGINE: OFF A/C CAB: OFF

HEPA SYSTEM: OFF A/C REAR: OFF

VENT FAN: OFF AMBULANCE:

STATIONARY 

7/26/95 4.0 3.1 1.1 3.5 2.9

ENGINE: ON A/C CAB: MAX,

RECIRC.

HEPA SYSTEM: ON A/C REAR: ON,

RECIRC.

VENT FAN: OFF AMBULANCE:STAT

IONARY 

7/26/95

7/26/95

34.4

45.6

45.1

47.4

53.6

51.8

39.1

41.4

43.0

46.6

ENGINE: ON A/C CAB: MAX,

RECIRC.

HEPA SYSTEM: ON A/C REAR: ON,

RECIRC.

VENT FAN: OFF AMBULANCE:

MOBILE 

7/26/95

7/27/95

41.5

41.7

48.5

45.4

41.4

50.4

48.4

47.9

45.0

46.4

ENGINE: ON A/C CAB: OUTSIDE

AIR

HEPA SYSTEM: ON A/C REAR: ON, OPEN

VENT FAN: HIGH AMBULANCE:

MOBILE 

7/27/95 60.9 58.4 46.7 57.6 55.9

NON–HEPA AMBULANCE #109
TABLE 1

AIR CHANGE RATES BASED ON AEROSOL CLEARANCE IN TYPE III AMBULANCES WITH AND WITHOUT

SUPPLEMENTAL HEPA FILTRATION

HETA 95–0031

UNIVERSITY HOSPITAL

NEWARK, NEW JERSEY

Health Hazard Evaluation Report No. 95–0031 Page 7

ENGINE: ON A/C CAB: MAX,

RECIRC.

HEPA SYS: NOT PRESENT A/C REAR: ON,

RECIRC.

VENT FAN: OFF AMBULANCE:

MOBILE

7/27/95

7/27/95

7/27/95

31.3

30.6

23.7

24.2

32.5

29.9

22.3

34.8

30.2

25.6

30.3

26.8

25.8

32.0

27.6

ENGINE: ON A/C CAB: OUTSIDE

AIR

HEPA SYS: NOT PRESENT A/C REAR: ON,

OPEN

VENT FAN: MEDIUM AMBULANCE:

MOBILE 

7/27/95

7/27/95

28.6

24.4

30.1

0.85

36.2

29.6

29.8

29.2

31.2

27.7‡

†

AIR CHANGE RATES WERE CALCULATED BASED ON CLEARANCE OF PARTICLES 3.0 MICRONS OR GREATER IN SIZE. ‡

MEAN ACH DID NOT INCLUDE THE FAMILY MEMBER SEAT LOCATION BECAUSE THIS VALUE WAS AN OUTLIER.
Page 8 Health Hazard Evaluation Report No. 95–0031

Figure 2 -- Particle counts vs. time for 

Ambulance 113 

engine off

(ACH=3.5)

engine, HEPA 

and A/C on

(ACH=39.1)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

14:42:2014:43:2614:44:3214:45:3814:46:4414:47:5014:48:5614:50:0214:51:0814:52:1414:53:2014:54:2614:55:32

Time 

Natural log of Particle 

Counts