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Study finds in-cabin particulate pollution up to 40% higher in traffic jams or at red lights

A new study by a team at the University of Surrey has 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.

The study, published as an open access paper in the RSC journal Environmental Science: Processes & Impacts, assessed in-cabin exposure to fine and coarse PM under five different ventilation settings and compared in-cabin exposure at signalized traffic intersections (TIs) with pedestrian exposure. The study also found that car windows closed with the fan/heating off in traffic is the best ventilation setting in traffic—leading up to a 76% reduction in in-car pollutants. Also, the safest setting is the air being circulated internally only by the fan without drawing in polluted air from outdoors.

For the study, the two researchers performed “mobile” and “fixed-site” monitoring of size-resolved particulate matter concentrations (PMCs) in the range of 0.25–34 μm at TIs. They made on-road mobile measurements inside a car under five different ventilation settings on a 6 km (3.7 miles) round route, passing through 10 different TIs. Fixed-site measurements were conducted at two types (3- and 4-way) of TIs.

They also estimated the zone of influence (ZoI) for PM10, PM2.5 and PM1 under different driving conditions and fitted the probability distribution functions to fixed-site data to understand the concentration and exposure dynamics of coarse and fine particles around the studied (3- and 4-way) TIs.

Among their other conclusions:

  • The in-cabin concentration of coarse and fine particles was affected differently by the air exchange rate (AER). Concentrations of coarse particles (PM2.5–10) decreased with an increase in AER; concentrations of fine particles increased.

  • Median PM10, PM2.5 and PM1 concentrations during delay periods at TIs were up to 40, 16 and 17% higher than those during free-flow conditions, indicating that TIs become hot- spots of PMCs during delay conditions.

  • The results showed the existence of a ZoI within the vicinity of a TI and that the length of a ZoI depends on the type of TI, fraction of PM, and traffic driving conditions. For stop-and-go driving conditions, the ZoI was found to depend on the average acceleration and driving speed of traffic.

  • Based on the fitting of the probability distribution function to the PM data at the fixed site, the hourly averaged PM10, PM2.5 and PM1 concentrations over the entire fixed-site measurements at the 3-way TI were found to exceed their corresponding values during free flow traffic conditions from mobile measurements for 41%, 34% and 35% of the total monitoring duration, respectively. The corresponding exceedances at the 4-way TI were 24%, 35% and 40%, respectively. It indicates that the frequency of exceedance increases with a decrease in the size of the particles.

  • On an average, only about 7% of the commuting time spent under delay conditions at TIs over all the runs was found to contribute 10, 7 and 8% of the total commuting exposure to PM10, PM2.5 and PM1, respectively. This indicates that TIs become hotspots of PM during delay conditions.

  • Exposure to on-road PM10 under delay conditions at the TIs was 6.2- and 7.3-times higher than that for a pedestrian at 3- and 4-way TIs, respectively. The corresponding ratios for PM2.5 were 1.4 and 1.2 at 3- and 4-way TIs, respectively.

  • Windows closed, fan and heating switched off under delay conditions was found to be the optimal ventilation setting for in-cabin exposure at the TIs, leading to the highest reduction in the in-cabin respiratory deposition dose (RDD) rate of PM1 (76%) and significant reduction in the in-cabin RDD rate of PM10 (42%) and PM2.5 (59%) with respect to the on-road RDD rate.

Where possible and with weather conditions allowing, it is one of the best ways to limit your exposure by keeping windows shut, fans turned off and to try and increase the distance between you and the car in front while in traffic jams or stationary at traffic lights. If the fan or heater needs to be on, the best setting would be to have the air re-circulating within the car without drawing in air from outdoors. Of course improving the efficiency of filtering systems of vehicles in future could further benefit to curtail the on-road exposure in such situations.

—Dr Prashant Kumar, senior author

Last year, a group of researchers led by Dr Kumar showed that drivers stuck at traffic lights were exposed up to 29 times more harmful pollution particles than those driving in free flowing traffic.


  • Kumar, P., Goel, A., (2016) “Concentration dynamics of coarse and fine particulate matter at and around signalised traffic intersections” Environmental Science: Processes & Impacts doi: 10.1039/C6EM00215C

  • Goel, A., Kumar, P. (2015) “Characterisation of nanoparticle emissions and exposure at traffic intersections through fast–response mobile and sequential measurements” Atmospheric Environment, 107, 374-390 doi: 10.1016/j.atmosenv.2015.02.002



And that is why you want to discourage diesel cars and trucks in towns and encourage electrics (including hybrids)*.
Not just for the CO2, but mainly for the local pollution effects.
*+ bikes, public transport, etc.


This has been going on for 100+ years but nobody measured it with enough accuracy? ICEVs, special diesels, have been playing with our health for over a century.


I always use recirculate when in traffic. The only problem is in traffic for a long time and human VOCs... ;)


Or buy a Tesla and use bio-defense mode!

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