['644', 'P.', 'A.', 'VALBERG', 'AND', 'A.', 'Y,', 'WATSON', 'hospital', 'and', 'varied', 'among', 'hospitals.', 'in', 'general,', 'mean', 'levels', 'of', 'indoor', 'PM', 'were', 'lower', 'than', 'those', 'measured', 'outdoors;', 'mean', 'values', 'indoors', 'were', 'approximately', 'one-fourth', 'to', 'one-half', 'those', 'measured', 'with', 'the', 'stationary', 'ambient', 'monitors.', 'Finally,', 'the', 'outdoor', 'central', 'monitor', 'did', 'not', 'predict', 'indoor', 'PM', 'concentrations.', 'Unfortunately,', 'the', 'authors', 'did', 'not', 'provide', 'any', 'information', 'on', 'occupancy', 'and', 'activity', 'levels', 'in', 'the', 'sampled', 'rooms.', 'if', 'ter-', 'minally', 'il', 'patients', 'are', 'dying', 'in', 'environments', 'with', 'minimal', 'PM', 'exposure,', 'uncorrelated', 'to', 'outdoor', 'PM,', 'then', 'how', 'can', 'ocutdoor', 'PM', 'be', 'causal?', 'Individual', 'Versus', 'Community', 'Mean', 'Exposures', 'In', 'many', 'of', 'the', 'PM', 'epidemiologic', 'studies,', 'outdoor', 'levels', 'of', 'PM', 'were', 'measured', 'using', 'a', 'sin-', 'gle', 'central', 'monitor.', 'However,', 'numerous', 'studies', 'have', 'demonstrated', 'that', 'ambient', 'PM', 'concentrations', 'at', 'central', 'monitors', 'are', 'not', 'representative', 'of', 'personal', 'exposures;', 'correlation', 'coefficients', '(R?)', 'range', 'from', '.004', 'to', '.34', '(Ozkaynak', 'et', 'al.,', '1996).', 'Because', 'the', 'epidemiologic', 'studies', 'are', 'popula-', 'tion', 'based,', 'we', 'do', 'not', 'have', 'personal', 'exposure', 'information', 'on', 'the', 'poten-', 'tially', 'susceptible', 'subpopulations.', 'Furthermore,', 'the', 'relative', 'risk', 'for', 'acute', 'mortality/morbidity', 'attributed', 'to', 'ambient', 'PM', 'is', 'only', 'a', 'few', 'percent.', 'The', 'correlations', 'emerge', 'only', 'after', 'complex', 'analyses,', 'averaging,', 'and', 'removal', 'of', '“long-term”', 'trends.', 'Because', 'of', 'the', 'acute', 'nature', 'of', 'the', 'response', 'and', 'the', 'proportionally', 'few', 'persons', 'affected,', 'it', 'would', 'seem', 'that', 'only', 'those', 'persons', 'experiencing', 'unusual', 'PM', 'exposures', 'are', 'of', 'interest.', 'Yet', 'fluctuations', 'in', 'ambient', 'PM', 'that', 'have', 'been', 'associated', 'with', 'the', 'acute', 'effects', 'are', 'small', 'compared', 'to', 'the', 'magnitude', 'and', 'fluctuations', 'in', 'levels', 'that', 'can', 'occur', 'with', 'indoor', 'PM.', 'With', 'such', 'low', 'relative', 'risks,', 'individual', 'exposures', 'are', 'very', 'important', 'and', 'may', 'be', 'critical', 'in', 'the', 'identification', 'of', 'causal', 'factors.', 'For', 'example,', 'for', 'thase', 'persons', 'residing', 'with', 'smokers,', 'their', 'individual', 'personal', 'exposure', 'is', 'not', 'represented', 'by', 'community', 'values', 'obtained', 'from', 'a', 'stationary', 'outdoor', 'monitor.', 'In', 'the', 'U.S.', 'Environmental', 'Protection', 'Agency', '(EPA)', 'public', 'buildings', 'study,', 'PM;', 'levels', 'were', 'measured', 'in', 'two', 'homes', 'for', 'the', 'elderly', '(Sheldon', 'et', 'al.,', '1988a,', '1988b,', 'cited', 'in', 'U.S.', 'EPA,', '1996a).', 'In', 'one', "smoker's", 'apartment,', 'PM,', '=', '39', 'pg/m®', '(2-day', 'average);', 'in', 'another', 'smoker’s', 'apartment,', 'shared', 'by', 'two', 'smokers,', 'PM,', '=', '88', 'pyg/m’;', 'and', 'in', 'two', 'nonsmokers’', 'apartments,', 'PM,', '=', '9.4', 'pg/m?', 'and', '8.6', 'pg/m?.', 'Therefore,', 'even', 'people', 'who', 'live', 'in', 'close', 'proximity,', 'with', 'identical', 'out-', 'door', 'PM', 'levels,', 'will', 'experience', 'very', 'different', 'personal', 'PM', 'exposures.', 'Dose', 'Considerations', 'Cause', 'and', 'effect', 'must', 'be', 'linked', 'by', 'a', 'biologically', 'plausible', 'process.', 'It', 'has', 'not', 'been', 'possible', 'to', 'identify', 'the', 'mechanism(s)', 'by', 'which', '“generic”', 'PM', 'might', 'exert', 'the', 'effects', 'attributed', 'to', 'it.', 'Any', 'mechanism', 'of', 'action', 'must', 'recognize', 'the', 'very', 'small', 'fluctuations', 'in', 'daily', 'dose', 'within', 'the', 'overall', 'low', 'dose', 'of', 'ambient', 'PM.', 'The', 'U.S.', 'EPA', 'has', 'estimated', 'that', 'for', '50', 'pg/m?®', 'of', 'the', '“Philadelphia”', 'ambient', 'particulate', 'aerosol,', 'the', 'daily', 'deposition', 'in', 'the', 'alveolar', 'region', 'was', 'about', '50', 'pg', '(U.S.', 'EPA,', '1996b).', 'The', 'local', 'dose', 'of', 'deposited', 'particles', 'to', 'lung', 'alveolar', 'tissues', 'can', 'be', 'estimated', 'from', 'the', 'fact', 'that', '50', 'pg', 'represents', '10°', 'unit-density,', '1-pm-diameter', 'particles', '(each', '0060082802']