MS BY PHYSICAL

sage of the substance through a filter with 0.2 to 0.45 |xm diameter pores. Many pharmaceuticals, such as solutions that are administered intravenously to pa­tients, are sterilized by passage through such filters. Bacteria and other living organisms are eliminated from the solution by trapping them on the filter, but viruses and some very small bacteria may pass through the filter.

Microorganisms can also be removed from air by passage through high efficiency particulate air fil­ters (HEPA filters), which remove particulate mater­ial larger than about 0.3 |xm. Clean rooms, such as operating theaters and rooms where drugs are pack­aged, often employ HEPA filters. Many microbiology laboratories also have laminar flow hoods in which air that is filtered through a HEPA filter is blown across the work area to prevent contamination dur­ing culturing of microorganisms. While not nearly as effective as a HEPA filter, wearing a face mask helps decrease the exchange of microorganisms between people. Surgical staff wear face masks to prevent ex­haling microorganisms into the open surgical wound. Staff and visitors wear masks when they are with patients who have infections that may be trans­mitted through the air. These precautions generally are adequate. However, greater precaution may be warranted for contact with patients with tuberculo­sis, and it is now required in many situations that a respirator with a HEPA filter be worn rather than a simple surgical mask.

Regardless of whether filtration or other methods, discussed below, are used to eliminate microorgan-

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