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138 Appendix B test and the atmospheric dust spot test. The atmospheric dust spot test is the test used to determine the "efficiency " of an air cleaner. The values obtained with these two tests are not comparable. For example, a filter with a weight arrestance of 90% may have an efficiency by the atmospheric dust spot test below 40%. The weight arrestance test is generally used to evaluate low efficiency filters designed to remove the largest and heaviest particles; these filters are com- monly used in residential furnaces and/or air-conditioning systems or as upstream filters for other air cleaning devices. For the test, a standard synthetic dust is fed into the air cleaner and the proportion (by weight) of the dust trapped on the filter is determined. Because the particles in the standard dust are relatively large, the weight arrestance test is of limited value in assessing the removal of smaller, respi- rable-size particles from indoor air. The atmospheric dust spot test is usually used to rate medium efficiency air cleaners. The removal rate is based on the cleaner's ability to reduce soiling of a clean paper target, an ability dependent on the cleaner removing very fine particles from the air. However, it should be noted that this test addresses the overall effi- ciency of removal of a complex mixture of dust, and that removal efficiencies for different size particles may vary widely. Recent studies by EPA, comparing ASHRAE ratings to filter efficiencies for particles by size, have shown that efficien- cies for particles in the size range of 0.1 to 1 microgram are much lower than the ASHRAE rating. Important features of this ASHRAE standard include: ■ definitions of arrestance and efficiency ■ establishment of a uniform comparative testing procedure for evaluating the performance of air cleaning devices used in ventilation systems ■ a discussion of the additional environmental parameters that must be considered ■ recommendations for summer and winter comfort zones for both temperature and relative humidity ■ a guideline for making adjustment for activity levels ■ guidelines for making measurements It should be noted that space tempera- tures above 76°F but within the summer comfort envelope have nevertheless been associated with IAQ complaints in offices. Note: As of summer 1991, a revised Standard 55 was nearly ready. Standard 52-76, "Method of Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter" This standard is intended to assist profes- sionals in the evaluation of air cleaning systems for particle removal. Two test methods are described: the weight arrestance test and the atmospheric dust spot test. The standard discusses differ- ences in results from the weight arrestance This air washer is used to remove particles and water-soluble gaseous contaminants and may also control temperature and humidity in the airstream. Such systems are subject to severe bacterial contamination.