The Hot Zone Book

From the text:

The suit has a clear, soft plastic bubble for a helmet. The suit is pressurized. Air pressure is supplied by an electric motor that sucks air from outside and passes it through virus filters and then injects it into the suit. This keeps the suit under positive pressure, so that any airborne virus particles will have a hard time flowing into it. A Racal suit performs the same job as a heavy-duty Chemturion space suit. It protects the entire body from a hot agent, surrounds the body with superfiltered air. pg. 176