Daily commuting accounts for approximately half of the average motorist’s exposure to harmful diesel particulate emissions, a new study says.

“If you have otherwise healthy habits and don't smoke, driving to work is probably the most unhealthy part of your day,” says Scott Fruin, an assistant professor of environmental health at the Keck School of Medicine at the University of Southern California, which conducted the study.

“Urban dwellers with long commutes are probably getting most of their exposure (to diesel and ultra-fine particles) while driving.”

Ultra-fine particulates, which are more toxic than larger particles, are of the greatest concern because they can penetrate cell walls and disperse throughout the body, he says.

The research team studied drivers in Los Angeles between February and April 2003 and found about half of their exposure to pollutants was during their drives to and from work.

For conducting the study’s measurements, the USC team used an electric vehicle with nine air pollution-monitoring instruments, along with a video camera for recording freeway traffic and driving conditions in the area.

In the 1.5 hours average drivers spend in their cars every day, they are exposed to 33%-45% of the harmful air pollution they breathe in, Fruin says, noting the greatest concentration of emissions comes from heavy trucks.

Hard acceleration, on both surface streets and in freeway driving, produced the greatest exposure to diesel pollution.

“The extent that (diesel trucks) dominated the highest concentration conditions on freeways was unexpected,” Fruin says. “Shortening your commute and spending less time in the car will significantly reduce your total body burden of harmful pollutants.”