Environmental Sampling. Environmental testing to detect B. anthracis on surfaces or in the air can be used to investigate known or suspected exposure events. The highest priority of an investigation is to evaluate the risk for exposure to aerosolized B. anthracis spores. Persons collecting and testing samples should 1) obtain adequate samples, 2) avoid cross-contamination during processing, and 3) ensure proficient laboratory testing and interpretation of test results. A positive laboratory test for B. anthracis from a sample of an environmental surface may be caused by cross-contamination from an exposure vehicle (e.g., contact with an envelope containing B. anthracis), background occurrence of B. anthracis spores in the environment, or previously aerosolized B. anthracis that has settled onto environmental surfaces. Laboratory test results of environmental surface samples should not be the only criterion for starting, continuing, or stopping antimicrobial prophylaxis for inhalational disease.