A single aerial application of the microbial insecticide "Foray48B®", a brand name for Bacillus thuringiensis var. kurstaki (commonly called Btk) certified by the Organic Materials Review Institute (OMRI), a national nonprofit organization that approves products used on operations that are certified organic under the USDA National Organic Program. . The active ingredient is a naturally occurring soil bacterium that when eaten kills gypsy moth caterpillars. Btk is specific to Lepidoptera species of caterpillars that ingest it and will not harm humans, bees, birds, fish, pets, and other wildlife.
Btk is highly unlikely to cause any adverse health reactions. It is possible, but very unlikely, that very few people with severe food allergies or asthma may react to the spray. If you suffer from such conditions,common sense suggests that you leave the area or take the simple precaution of staying indoors with the windows closed while the planes are spraying to avoid direct exposure and minimize your risk of reactions. More information is available online (PDF).