A surgical mask is designed to be worn by health professionals during surgery and at other times to catch the bacteria shed from the wearer's mouth and nose.
Outside health care facilities, simple, inexpensive surgical masks are commonly worn in heavily polluted various industrial centres in East Asia. More recently, they were widely used in China, Hong Kong, and Vietnam during outbreaks of the SARS virus.
Construction
Most "real" surgical masks used in operating rooms for the purposes of reducing infection are made of paper, and are disposed off after each use. Recently, some hospitals have started to re-introduce re-usable cloth masks, that may be cheaper over a long period of time.
Effectiveness
Simple surgical masks do reduce the spread of bacteria and aerosols. Apart from protecting the wearer from spashes in the mouth with body fluids they are designed to protect others from the wearers oral bacteria. They are not designed for protecting the wearer from inhaled particles.
Viral particles are far too small to be caught in the fibres of a regular surgical mask. Thus, a mask wearer would not be less likely to catch a viral disease than someone not wearing a mask. However, an already infected person wearing a mask may slightly reduce his chances of infecting others, as it may catch droplets of fluid expelled during sneeze or cough.
A surgical mask will trap some dust but is a lot less effective than masks designed for this purpose.
