Don't try to clean up the powder. Cover the powder or letter with something like clothing, paper, a plate, a sealed plastic bag, or a trashcan. Don't remove this cover. Leave the room and close the door. Wash your hands with soap and water. This will remove all spores from the skin surface. If your face or clothing was contaminated, remove clothing and place it in a plastic bag or other sealed container. Shower with soap and water as soon as possible.
In either type of exposure, contact your local police department to learn what to do next. If you are at work, contact your supervisor or your security office. If the exposure is verified, preventive treatment with antibiotics will be advised until testing of the substance is complete.
If you have symptoms of an illness that you suspect may be anthrax, contact your doctor, public health clinic, or local emergency department right away for further advice. If you think you were exposed but are having no symptoms of illness, contact your doctor or the emergency department once cleanup and reporting has been completed. You may need to take antibiotics for prevention.
For the latest information, see the CDC website. Or contact your state public health department.