Response: Window air conditioners contain rubber gaskets that can become brittle after many years of use. As a result, the rubber gaskets can eventually break apart into tiny pieces that can then be blown out of the air conditioner. If your air conditioner is more than several years old, the black dust is probably these little pieces of gasket that are blown from your air conditioner.
These gasket pieces are probably not dangerous. Since you can see the specks of the gasket, the gasket pieces are too big for you to breathe them into your lungs. It is possible that little gasket pieces can get into your body if they get blown on your food or if they get on your hands and you happen to eat them by mistake. But, eating these gasket pieces by accident are unlikely to be cause you any harm. Although not hazardous to your health, it makes sense that you will want to either fix the problem or get a new air conditioner. Fixing the problem should be as simple as replacing the problem gasket or gaskets. You may be able to get a replacement gasket through your air conditioner manufacturer or at your local hardware store. Your air conditioning manufacturer may also have a technical support line that could tell you how to check for leaking freon. If you do have a freon leak, however, you might have very little or no freon in your air conditioner, especially if your air conditioner has been sitting around for a while. If so, your air conditioner will no longer blow cold air -- a pretty sure sign that something is wrong!
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Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to Helen@TreeHugger.com