Diesel fumes suppress immune response.

Recurring exposure to soot particles from diesel exhaust reduces the immune system's capacity to fend off infection more persistently than does a one-time exposure to an equivalent amount of particles, tests on rodents indicate.

Inhaling particles less than 2.5 micrometers across is harmful to the heart and lungs. A past study showed that breathing air filled with such emissions for 4 hours temporarily suppressed rats' immune defenses against the bacterium Listeria monocytogenes, within a week after being deliberately infected with the bacterium, however, the soot-exposed rats cleared the infection as effectively as did animals that hadn't breathed diesel fumes.

In follow-up research, Joseph K.H. Ma of West Virginia University in Morgantown and his colleagues gave rats the same dose of diesel-derived particles as had been administered in the earlier study, but they spread the exposure over 4 hours on each of 5 consecutive days. In this scenario, which the researchers consider more like people's diesel-fume exposure in cities, more Listeria survived for longer than a week in the lungs of diesel-exposed animals than in rats that breathed clean air.

Chronic exposure to diesel particles appears to impair the immune system's function more than intermittent, acute exposures do, the researchers conclude in the February Toxicological Sciences.--B.H.