ICTA 2000 - “In-Car Air Pollution: The Hidden Threat to Automobile Drivers"

International Center for Technology Assessment,
“In-Car Air Pollution: The Hidden Threat to Automobile Drivers"
Report No. 4, An Assessment of the Air Quality Inside Automobile Passenger Compartments
Washington, DC: July 2000
On the Web
Relevance: high

This report reviews 23 studies from between 1982 and 1998 covering the main pollutants inside cars: particulate matter, volatile organic compounds, carbon monoxide, nitrogen oxides, and ozone.  For all exhaust pollutants except CO and the largest PM, concentrations are typically higher inside cars in heavy traffic than elsewhere.

Particulate Matter (PM)
Health Effects: PM can cause irritation, aggravate respiratory conditions, and possibly premature death.  Fine PM seems even scarier because it can carry known carcinogens and invade the lungs deeper than larger PM.

Exposure: A 1994 US study by Ptak and Fallon found that 90% of PM inside vehicles measured less than 1 micrometer across (PM10 is defined at PM with diameter up to 10 micrometers and PM2.5 has a diameter up to 2.5 micrometers.  The EPA defines PM with diameter 10-2.5 micrometers as coarse, diameter 2.5 or smaller as fine, and diameter 0.1 or smaller as ultrafine.).  The study measured average in-car PM concentrations of 1.5 micrometers/m^3 during highway driving conditions.  Air conditioning systems can remove 40-75% of large PM but only 2-15% of PM 1 micrometers or smaller.

Volatile Organic Compounds (VOCs)
Health Effects: Some VOCs, such as benzene, have a carcinogenic affect associated with cumulative exposure.  Children exposed to benzene have a much higher risk of leukemia than do adults, even at low ewer exposure levels.  The WHO’s acceptable exposure level for benzene is zero.  1,3 butadiene and formaldehyde are “probable human carcinogens” according to the EPA.  VOCs also form ozone (a.k.a. smog) when exposed to sunlight.


  • concentrations to be 6.1 times higher in cars than at fixed-site measuring stations and 1.7 times higher than at the side of the road.  Concentrations inside and immediately outside vehicle were roughly the same.
  • A second 1991 Harvard study in Boston found that despite car commutes being shorter than subway commutes (76 vs. 87 mins.), car commuters were exposed to more benzene and some other VOCs.  Total VOC exposure for walkers (47 mins) and bikers (54 mins) was even lower.  The study also calculated that the daily commute accounted for approximately 21% of car commuters’ daily benzene exposure (less than 4% of your day = more than 20% of your benzene exposure).  Train commuters got 10% of their daily benzene from the commute (don’t know whether their total daily benzene exposure was less than that of car commuters).
  • Early US studies (late 1980’s-early 1990’s) measured benzene concentrations in passenger cars ranging from 13.6 to 50.4 micrograms/m3; toluene from 33.3 to 158.0 micrograms/m3; ethylbenzene, 5.8 to 11.6 micrograms/m3; m&p-xylene, 20.9 to 154.0 micrograms/m3; o-xylene, 7.3 to 16.0 micrograms/m3; and formaldehyde, 0.2 to 13.7 micrograms/m3.  Most of the high ranges are from Los Angeles and the lowest from Raleigh, NC and Boston.  In two studies in-car concentrations were higher during urban driving than during suburban or highway driving.  One study showed that improperly maintained vehicles have higher in-vehicle concentrations.
  • A 1991 Harvard School of Public Health study measured average benzene
  • A 1995 study in Paris compared in-car pollutant levels of a gas and electric vehicle, finding that levels were similar so most in-car VOCs come from outside exhaust not self-pollution (except from smoking).  The study also estimated that non-smokers get 20-30% of daily benzene from commuting.
  • A 1998 CARB study confirms that VOC concentrations are higher inside cars than at the roadside or at remote fixed-sites and are similar to concentrations in the traffic stream.  In-car benzene levels in Sacramento during rush hour ranged from 10.3 to 13.9 ?g/m3 while roadside measurements ranged from 2.6 to 5.0 ?g/m3.  LA had less dramatic differences between in-car and roadside levels.

Carbon Monoxide (CO)
Health Effects: Carbon monoxide combines with hemoglobin and prevents it from supplying oxygen to the brain and other tissue.  CO binds 200-230 times more readily to hemoglobin than does oxygen and can damage the hemoglobin so that it never delivers oxygen again.  CO levels in cars rarely exceed federal standards, but there is some concern with chronic exposure, although this is not yet well understood.


  • Research shows that CO levels inside cars are consistently higher than in the ambient air.
  • A 1991 Harvard study measured overall (urban, interstate, rural) average in-car CO at 11.3 ppm, immediately outside the car at 11.7 ppm, and nearby fixed-site at 2.9 ppm..  On urban streets, interstates, and rural roads the median concentrations were 13, 11, and 4 ppm, respectively.
  • The 1998 CARB study measured in-car CO levels in LA and Sacramento under a variety of driving conditions.  Important factors included following polluting vehicles and traffic density.  See that paper for details.
  • 1995 Paris study found concentrations at pedestrian sidewalks were approximately 3 times lower than in the cars on the street.
  • 1995 Mexico City study showed in-car CO levels (56.1 ppm) were more than 5 times ambient levels, interesting because Mexico City’s ambient levels are already high at 7.2-11.3 ppm.

Nitrogen Oxides (NOX)
Health Effects: NOX is an irritant that can exacerbate respiratory diseases and reduce the lungs ability to resist viruses and bacteria (like the cold, flu, and pneumonia).  Exposure to concentrations of less than 30 ppb is associated with bad stuff and concentrations of around 80 ppb have been correlated with increased respiratory infections and sore throats, especially in children.

Exposure: A 1995 study in Amsterdam measured in-car concentrations of NO2 at less than 31 ppb on a rural route and average in-city in-car concentrations from 31 to 90.8 ppb.  In-city bikers breather 49.6 to 81.4 ppb and pedestrians breathed 55.3 ppb, on average.

Ground Level Ozone
Health Effects
: Ground-level ozone, also known as smog, damages lung tissues and exacerbates existing respiratory diseases.  Studies indicate that hospitalizations for certain respiratory ailments increase by 6-10% for every 50 ppb increase in peak ozone exposure.  Ozone is created when VOCs and NOX in exhaust react with sunlight.

Exposure: Limited research indicates that ozone levels are lower in vehicles that in the ambient air, possibly because exhaust enters the vehicle before much ozone has a chance to form and other reasons. (This report also contains a really great graph in the conclusion showing emissions per person for various modes of transportation.  A single-occupant car emits 209 times more VOCs per person/mile than a transit train and 10.5 times more than a transit bus, for example.


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Eric de Place

1. The benzene findings seem critical to me. However, it would be good to find some additional context for evaluating exposure levels. WHO says no exposure is acceptable. But can we characterize, say, 10 micrograms of benzene as "increasing the risk of x"? And is 20 micrograms twice as bad, or worse?

2. I'd be wary of using international research. Emissions and maintenance standards are far different in Mexico than in the US. Even in Europe there are big differences, partly because of the drastically higher use of diesel engines. (On that note, it would be good to figure out which emissions are higher from diesel--like PMs.)

3. Love the ground-level ozone comparison. It would be neat to come up with a "personal smog footprint" for people based on their travel habits.

Tonya Watts

Does this information consider closed or open windows?

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