Office bustle launches anthrax spores. (Dust Up).

The commotion of everyday business can give anthrax spores a second wind, a new study suggests. Normal office activity stirs up the dangerous particles parked on contaminated indoor surfaces and sends them into the air.

After letters packed with the bacterium Bacillus anthracis passed through several buildings in the eastern United States in the fall of 2001, anthrax infections killed 5 people and sickened 17 others.

In an office suite in the Hart Senate Office Building in Washington, D.C., while it was closed after workers received an anthrax containing letter, government investigators set out to assess the environmental risks. The tainted letter had scattered spores onto surfaces and into the air in the suite where it was opened.

Christopher P. Weis of the Environmental Protection Agency in Denver and his colleagues conducted three sets of tests in the Hart building offices over a week in November 2001. During the first of their trials, they minimized movements that might create air currents within the suite. For subsequent tests, however, they simulated normal workday activities such as sorting and handling documents, pacing, and moving chairs, trashcans, and other objects.

Meanwhile, the researchers collected clumps of B. anthracis spores as they settled onto stationary lab dishes or were sucked into air filters that the scientists set on the floor or mounted on their protective gear. The team also vacuumed up spores from carpets and other surfaces.

The researchers recovered many more airborne spores under simulated workday conditions than under calmer circumstances. More than 80 percent of the airborne clumps of spores were 3.5 micrometers or smaller in diameter. Such small clusters remain airborne longer than larger clumps and are more likely, if inhaled, to penetrate deep into the lungs, where B. anthracis is most dangerous.

The team's results, which appear in the Dec. 11 Journal of the American Medical Association, indicate "how lucky we were that so few people became ill," says Martin Hugh-Jones of Louisiana State University in Baton Rouge, who investigated a 1979 anthrax outbreak in Russia.

Matthew Meselson of Harvard University says the EPA researchers "have shown that there's a residual hazard" in contaminated indoor spaces. However, he adds, it shouldn't be assumed that if B. anthracis was released outdoors by terrorists, it would easily lift back into the air after landing on the ground.

Past studies of outdoor sites contaminated by naturally occurring B. anthracis haven't suggested that much of the pathogen returns to the air, notes Weis. He attributes the unexpected behavior of the spores in the Hart building to the small size of the clumps created by the letter's sender.

Another factor that could account for the new findings is the smaller average size of grains of normal indoor dust compared with outdoor soil particles, says Kenneth S.K. Chinn, a retired weapons researcher for the U.S. Department of the Army who lives in Dugway, Utah. Small particles are more likely than large ones to become suspended in air currents, where spores clinging to them could detach and get inhaled, Chinn says.

The ease with which spores rise into indoor air emphasizes the importance of thoroughly evaluating decontamination efforts, says Bill Kournikakis of Defence Research and Development Canada-Suffield in Medicine Hat, Alberta. Air-quality tests done under calm circumstances might turn up no apparent threat in a space that could nevertheless become a hot zone as soon as it's reoccupied, he says.