There are only intake filters on the air going to MV engines, not to the vent, which is the air you breath in side the vehicle. The difference between bicycling and motoring is that motorists aren't exercising on the road, so they're not breathing as deeply as a cyclist.
Diesel fumes are particularly noxious, and have not been adequately regulated due to the strong trucking industry lobby, which turned back diesel emission standards when congress passed laws regulating the emissions from other motor vehicles, way back in the 70's. Cleaner diesel emissions are still a few years off. Your nose doesn't filter diesel particulates from the air, see below:
From NRDC,
http://www.commondreams.org/news2003/0610-13.htm :
Why EPA Must Reduce Diesel Emissions:
Particulate soot from diesel engines has been linked with increased asthma attacks and emergencies, cancer, endocrine disruption, heart ailments, and premature death.
Virtually all diesel particulate soot is less than one micron in diameter, i.e., small enough to evade our respiratory defenses and lodge in the deepest recesses of our lungs.
The U.S. EPA and the World Health Organization's cancer research agency agree that diesel exhaust is likely to cause cancer.
In midtown Manhattan, diesel engines emit more than half of the PM inhaled by New York pedestrians, even though gasoline vehicles outnumber diesel vehicles by ten-to-one in New York State.
Diesel nitrogen oxide (NOx) emissions contribute significantly to the nation's chronic summertime smog (or ozone) and acid rain, as well as to water pollution in the NYC drinking water system and other large water bodies, crop damage and forest impacts.
More than 82 million Americans live in areas that do not meet EPA's upcoming health standard for particulate soot. More than 122 million Americans live in areas that do not meet EPA's health standard for daily exposure to ozone smog.
More FAQs on diesel emissions:
http://www.nett.ca/faq_diesel.html