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USC researchers find car ventilation setting critical to in-cabin exposure to particulate pollutants; new model for aiding exposure assessments

In-vehicle-to-outside (I/O) ratios for four pollutants under different ventilation settings. Credit: ACS, Hudda and Fruin (2013). Click to enlarge.

Researchers at USC have found that using recirculation rather than outside air ventilation in a car can effectively reduce in-cabin exposure to on-road particle pollution. In a study published in the ACS journal Environmental Science & Technology, they also report that in addition to the benefits of recirculation settings, exposures are lower in newer cars; at slower speeds; and on arterial roads, where pollutant concentrations are lower than on freeways.

Scott Fruin, assistant professor of preventive medicine, and Neelakshi Hudda, research associate in environmental health, both of the Keck School of Medicine of USC, performed a systematic study of in-vehicle exposure to include a full range of car types and operating conditions, as well as all types of particulate pollution. According to the researchers, concentrations of particle pollutants on freeways are often five to 10 times higher than elsewhere.

Traffic-related particulate pollutants studied included ultrafine particles to the larger particulate matter (PM) sizes such as PM2.5 that are linked to heart disease and premature mortality. Pollutants also included black carbon and particle-bound polycyclic aromatic hydrocarbons, both high in diesel emissions, considered by the state of California to produce cancer.

Under closed-window driving conditions, the in-vehicle-to-outside (I/O) concentration ratio for traffic-related particulate pollutants ranges from nearly 0 to 1 and varies up to 5-fold across a fleet of vehicles, thus strongly affecting occupant exposures.

Concentrations of five particulate pollutants (particle-bound polycyclic aromatic hydrocarbons, black carbon, ultrafine particle number, and fine and coarse particulate masses) were measured simultaneously while systematically varying key influential parameters (i.e., vehicle type, ventilation, and speed). The I/O ratios for these pollutants were primarily determined by vehicle air exchange rate (AER), with AER being mostly a function of ventilation setting (recirculation or outside air), vehicle characteristics (e.g., age and interior volume), and driving speed. Small (±0.15) but measurable differences in I/O ratios between pollutants were observed, although ratios were highly correlated.

—Hudda and Fruin (2013)

By random sampling, Fruin and Hudda found that for a typical car (the national average is 7 years old), recirculation settings reduce in-vehicle particle pollution for very small particles from 80% (of on-road levels) to 20%, and from 70% to 30% for larger particles, compared to air ventilation settings, which bring in outside air.

AF (attenuation factor) dependence on AER for 25–400 nm particles under recirculation (RC) and outside air (OA) ventilation settings. AF is a measure of particle removal rather than particle persistence within the vehicle cabin. Credit: ACS, Hudda et al. (2011) Click to enlarge.

(Windows were always assumed to be closed for this test. Keeping windows open while driving quickly raises inside pollutant concentrations to the same levels as on-road levels.)

However, the researchers found that leaving the windows closed over 30-minute or longer drives with several passengers also raised carbon dioxide (CO2) levels in compact, new cars, matching the levels found in stuffy meeting rooms.

Fruin and Hudda used this study to build on prior studies of ultrafine particle number I/O ratios to develop predictive models for other particulate pollutants. They found that the models explained more than 60% of measured variation, using ventilation setting, driving speed, and easily obtained vehicle characteristics as predictors.

The models developed in this study are useful for allowing estimation of I/O ratios for PB-PAH, PM2.5, and PM10 (BC I/ O ratios appeared to be similar to those for PB-PAH).

...It is not feasible in large cohort studies to conduct in-vehicle measurements for all subjects, and in smaller panel studies, it is difficult to measure in-vehicle exposures for multiple pollutants. Therefore, our models that can predict I/O ratios for multiple pollutants based on information that can be gathered through questionnaires can aid exposure assessment.

—Hudda and Fruin (2013)


  • N. Hudda and S. A. Fruin (2013) Models for Predicting the Ratio of Particulate Pollutant Concentrations Inside Vehicles to Roadways. Environmental Science & Technology doi: 10.1021/es401500c

  • Neelakshi Hudda, Evangelia Kostenidou, Constantinos Sioutas, Ralph J. Delfino, and Scott A. Fruin (2011) Vehicle and Driving Characteristics That Influence In-Cabin Particle Number Concentrations. Environ. Sci. Technol. 45 (20), pp 8691–8697 doi: 10.1021/es202025m



I'm thinking it might be worthwhile to change to in-car recirculation when going down the motorway, but OTOH particulates in the town centre here in the UK may be at different levels to in LA, and the milder climate here may disperse particulates better as soon as you are out of town on the motorway.
Around my way there are not many urban motorways.


Many would be surprised to know the extend of various pollution modern industries, cars, hospitals etc are throwing at us.

My new car smell is so strong that it is making me sick. I have to drive with full exterior ventilation and that is not the best solution either?


"performed a systematic study of in-vehicle exposure to include a full range of car types and operating conditions, as well as all types of particulate pollution. "

I just confused about these words,since I live in beijing.It`s quite different issue between US and CHN,why concentrations of particle pollutants on freeways are often five to 10 times higher than elsewhere?


The human brain lowers blood flow to itself when through the nose it senses toxic materials. This essentially lowers brain function, as a self protection mechanism. Many humans actually like this feeling, thus the weird appeal to many of the toxic chemicals used in perfumes and colognes. Many like the smell of gasoline, burning materials and nitric oxides, because it makes their brains numb.


Should just mention, that I have for a long time known that recirculating air in your car as opposed to flow through air, at least in the short term, lowers your exposure to exhaust pollutants. You do need to be concerned with CO2 however. Typically, if I can, I wait for a low traffic area or other area where the normal pollutants are lower and then just flush the passenger compartment by temporarily opening a window.

Kit P

I take it all back thinking that California universities were full of dumb bunnies for professors. With a lot of research they have discovered you should put the A/C on recirc when driving past the hog farm.

They have also discovered that if you have clean air outside you will have clean air outside.

“that are linked to heart disease and premature mortality.”

The link is premature mortality for those with severe heart disease. The word ‘link’ is used when describing a half-baked theory. When the theory cannot be validated with evidenced after 15 years, that makes it junk science.

Kit P


“I just confused about these words, since I live in beijing.”

The words were intended to be confusing. At some universities, the environmental studies department presents the data in a confusing manner to scare the general public. I think this is dishonest and unethical.

Here is an example.

The air quality index is 15 where we are today. With a I/O ration of 0.5 it would be 7.5. In San Diego, California the air quality index is 64 and inside a car it could be 32. If the air quality index is 800 in Beijing and inside a car it could be 400.

If adverse health effects of are seen above levels of air quality index of 80 for prolonged exposure, pollution is not a problem where I am but is in Beijing. It is unethical to present data indicating there is a problem when none exists.

The good news for China is that the technology used in the US to reduce air pollution over the last 50 years is available to China.

Kit P

"You do need to be concerned with CO2 however. "

No, CO2 is not toxic. Adequate levels of O2 are needed which would only be a problem if you were driving a car inside a cargo plane.


When the vehicle on front is belching out a bit of smoke then, yes you switch the recirc function. We know that already don't we.

Hence isn't this a bit of an elaborate way of saying that that bears really do defecate in the woods?


@Kit P
Thank you for your information.

The good news for China is that the centre goverment announced a clear air program with 5 billion RMB at 14.Oct,the Environmental Protection Minister already put 500 million into his cognate`s pocket so far.

Our problem is not only equipment but also the people.

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