NEW YORK (Reuters Health) -- Vehicles cruising the nation's highways leave a high-speed mix of sneeze-inducing allergens in their wake, researchers report.

"Allergens from at least 20 different source materials were found in paved road dust," according to investigators led by Dr. Ann Miguel of the California Institute of Technology in Pasadena.

Reporting in the December 1st issue of Environmental Science & Technology, Miguel's team describes road dust as "a complex mixture of soil dust, deposited motor vehicle exhaust particles, tire dust, brake lining wear dust, plant fragments, and other biological materials."

They theorize that road dust -- swept into the atmosphere by passing traffic -- could be a strong contributor to allergy in urban areas.

To test this theory, the investigators vacuumed samples of road dust from three sites in Southern California.

They report that all of the samples contained substances "known to be capable of causing or exacerbating (allergies) in humans." In fact, Miguel and colleagues contend that stirred-up road dust may be the source of up to 12% of airborne allergens inhaled by residents living in the areas tested.

The researchers note that all of their tests took place in dry, thinly-vegetated Southern California. Because vegetation plays a strong role in increasing rates for airborne allergy, they speculate that road dust may be an even greater source of allergens "in areas with more abundant vegetation."

Risks for allergy and asthma linked to road dust may be greatest among those who live in close proximity to streets or highways, according to the researchers. They point to the results of a recent Dutch study, which found that "cough, wheeze, runny nose, and diagnosed asthma were significantly more often reported for children living within 100 meters (328 feet) from the freeway."