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WUSTL study finds use of air conditioning reduces in-car pollution

Traffic is a major source of harmful pollutants; daily peak exposures tend to occur near roadways or while traveling—or being stuck—on them. For example, a team at the University of Surrey found that particulate pollution levels inside cars are up to 40% higher when the vehicle is stuck in a traffic jam or stopped at a red traffic light compared to free-flowing traffic conditions. (Earlier post.)

A team at the University of Washington in St Louis has now measured simultaneous real-time particulate matter (particle numbers, lung-deposited surface area, PM2.5, particle number size distributions) and CO concentrations outside and in-cabin of an on-road car during regular commutes to and from work. Data was collected for different ventilation parameters (windows open or closed, fan on, AC on), while traveling along different road-types with varying traffic densities. They found that car drivers can expect their highest exposures when driving with windows open or the fan on, and their lowest exposures during windows closed or the AC on. Their paper is published in Atmospheric Environment.

Anna Leavey, a research scientist at the School of Engineering and Applied Science, and Nathan Reed, a PhD candidate, worked together with PhD candidate Sameer Patel and Pratim Biswas, the Lucy and Stanley Lopata Professor and chair of the SEAS department of Energy, Environmental and Chemical Engineering. With assistance from Biswas’s Aerosol and Air Quality Research Lab, they used portable instruments and sensors to monitor and simultaneously measure the pollutant levels of their car’s indoor cabin air and the air directly outside of the car during their own daily commutes. That gave them rare, real-world look at pollutant exposure.

Using their simultaneous measurement approach, Leavey and Reed were able to test a number of variables while driving to and from Washington University over a four-month period starting in 2014. Using a dashcam, they were able to identify a given pollutant concentration each time they were: stuck behind a bus or truck, amid traffic on a freeway, stopped at a red light, or driving past restaurants or construction work. They also used different ventilation settings inside their cars: driving with the windows open, windows closed, with fan on, and with the air conditioning on.

Among their findings:

  • Ambient pollutants (NOx, PM2.5, CO) and meteorological variables (wind speed, temperature, relative humidity, dew point) explained 5–44% of outdoor pollutant variability, while the time spent traveling behind a bus was statistically significant for PM2.5, lung-deposited surface area (SA), and CO.

  • Time-series analysis demonstrated that cabin concentrations tended to track measured outdoor concentrations albeit with some reduced variability and time-lags when windows were open.

  • The geometric mean diameter (GMD) for outdoor aerosol was 34 nm. Larger cabin GMDs were observed when windows were closed compared to open (b = 4.3, p-value = <0.01). When windows were open, cabin total aerosol concentrations tracked those outdoors.

  • With windows closed, the pollutants took longer to enter the vehicle cabin, but also longer to exit it.

  • Particle number concentrations were impacted the most by changes to window position/ventilation, and PM2.5 the least.

  • Commuting with the windows closed or the AC on reduced cabin concentrations significantly enough to disrupt cabin / outdoor correlations for the majority of measured pollutants, and frequently prevented the diffusion of CO into the vehicle. Running the AC also significantly increased the mean size of particle numbers measured inside the vehicle.

In general … car drivers can expect their highest exposures when driving with windows open or the fan on, and their lowest with windows closed or the AC running. A driver may be able to control their commuting exposures by being cognizant of their pollutant environment and applying dynamic behavior modification to adapt to changing scenarios.

—Leavey et al.

When the AC is operating, you have a cold evaporator that is cooling the air as it passes. This cold surface attracts the pollutant particles, and they deposit there, as opposed to diffusing it into the air you’re breathing.

—Nathan Reed

That particle deposition offered varying degrees of pollution protection, but was most boosted at points of elevated exposure during the commute, such as following a bus or large truck.

When windows were closed, and following a bus, the particle concentration in the outdoor air was 3 times higher than the indoor air. What’s more, no in-cabin carbon dioxide concentrations were measured during 75% of the journeys made with the AC on.

The vehicle cabin can be viewed as a buffer, protecting us from the outside air. While driving with your air conditioning on and windows closed is the most protective thing that you can do, running the AC can decrease your fuel economy. That’s why adopting a dynamic behavior modification approach is recommended, in which the AC or closed windows are used when following a highly polluting vehicle, or on the freeway which tends to be more highly polluted. Once you have left the polluted environment, we recommend opening your windows to remove any pollutant build-up from your car.

—Anna Leavey

This research was funded by the Lopata Endowment Fund and the McDonnell Academy Global Energy and Environmental Partnership (MAGEEP) at Washington University in St. Louis.


  • Anna Leavey, Nathan Reed, Sameer Patel, Kevin Bradley, Pramod Kulkarni, Pratim Biswas (2017) “Comparing on-road real-time simultaneous in-cabin and outdoor particulate and gaseous concentrations for a range of ventilation scenarios,” Atmospheric Environment doi: 10.1016/j.atmosenv.2017.07.016



In many cities with high pollution level, cars equipped with HEPA filter A/Cs could become a must, specially when driving behind diesel vehicles?

Full time use of HEPA A/C will reduce range on most BEVs but may be required for driver/passengers health?

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