['ALTERNATIVE', 'HYPOTHESES', 'FOR', 'PM', 'EFFECTS', '659', 'allergen.', 'Cookson', 'and', 'Moffatt’s', 'hypothesis', 'is', 'supported', 'by', 'concrete', 'data', 'linking', 'causes', '(infectious', 'agents)', 'with', 'mechanistic', 'factors', '(immune', 'cell', 'cytokines)', 'with', 'measures', 'of', 'atopy', '(symptoms,', 'allergen', 'specific', 'IgE', 'levels,', 'skin', 'tests).', 'This', 'example', 'relating', 'to', 'asthma', 'illustrates', 'the', 'critical', 'impor-', 'tance', 'of', 'considering', 'biologic', 'plausibility', 'and', 'mechanism', 'of', 'action.', 'Simi-', 'larly,', 'alternative', 'explanations', 'for', 'the', 'mortality/morbidity', 'results', 'should', 'be', 'evaluated', 'before', 'assuming', 'a', 'causative', 'role', 'for', 'ambient', 'PM.', 'SUMMARY', 'AND', 'CONCLUSIONS', 'A', 'number', 'of', 'recent', 'epidemiology', 'studies', 'have', 'reported', 'small', 'positive', 'associations', 'of', 'mortality', 'and', 'morbidity', 'with', 'ambient', 'levels', 'of', 'PM.', 'Al-', 'though', 'the', 'associations', 'reported', 'are', 'reasonably', 'coherent', 'and', 'consistent,', 'it', 'is', 'not', 'possible', 'to', 'attribute', 'causality', 'to', 'ambient', 'PM.', 'People', 'spend', 'the', 'majority', 'of', 'their', 'time', 'indoors', 'breathing', 'PM', 'that', 'may', 'be', 'unrelated', 'either', 'quantitatively', 'or', 'qualitatively', 'to', 'the', 'ambient', 'PM', 'measured', 'by', 'outdoor', 'monitors.', 'Lung', 'tissue', 'doses', 'for', 'the', 'ambient,', 'low', 'levels', 'of', 'PM', 'being', 'studied', 'are', 'so', 'small', 'as', 'to', 'undermine', 'the', 'biological', 'plausibility', 'of', 'ambient', 'PM', 'being', 'responsible', 'for', 'the', 'mortality/morbidity', 'attributed', 'to', 'it.', 'Considerable', 'energy', 'has', 'been', 'spent', 'on', 'examining', 'the', 'direct', 'associa-', 'tions', 'between', 'ambient', 'PM', 'and', 'health', 'effects,', 'with', 'minimal', 'attention', 'to', 'alternative', 'hypotheses.', 'Figure', '1,', 'however,', 'illustrates', 'other', 'possible', 'rela-', 'tionships,', 'with', 'causal', 'pathways', 'leading', 'to', 'increases', 'in', 'daily', 'mortality/', 'morbidity.', 'Human', 'activity', 'patterns', 'are', 'necessarily', 'linked', 'to', 'the', 'magni-', 'tude', 'of', 'anthropogenic', 'particulate', 'emissions', '(Hildemann', 'et', 'al.,', '1991).', 'Weather', 'fluctuations', 'affect', 'both', 'outdoor', 'PM', 'levels', 'and', 'the', 'airborne', 'levels', 'of', 'indoor', 'PM', 'toxins.', 'The', 'support', 'and', 'companionship', 'provided', 'by', 'friends', 'and', 'family', 'to', 'hospitalized', 'patients', 'are', 'correlated', 'to', 'mortality', 'risks', 'and', 'conceivably', 'to', 'ambient', 'PM.', 'Because', 'the', 'magnitude', 'of', 'the', 'reported', 'increases', 'in', 'mortality/morbidity', 'associated', 'with', 'ambient', 'PM', 'is', 'so', 'small,', 'it', 'would', 'take', 'minimal', 'confounding', 'by', 'alternative', 'causal', 'pathways', 'to', 'pro-', 'duce', 'the', 'reported', 'results.', 'For', 'example,', 'if', 'an', 'excess', 'of', 'only', '3', 'out', 'of', '100', 'deaths', 'occurred', 'during', 'a', '24-h', 'increase', 'of', '50', 'pym/m?®', 'PM', 'during', 'a', 'winter', 'cold', 'spell,', 'it', 'is', 'conceivable', 'that', 'those', '3', 'persons', 'could', 'have', 'turned', 'on', 'a', 'humidifier', 'for', 'household', 'winter', 'dryness,', 'started', 'up', 'a', 'nonvented', 'kerosene', 'stove', 'for', 'heat,', 'or', 'decided', 'to', 'vacuum', 'because', 'they', 'were', '“stuck”', 'indoors.', 'All', 'of', 'these', 'activities', 'generate', 'indoor', 'aerosols,', 'some', 'of', 'which', 'are', 'biolog-', 'ically', 'active', 'at', 'very', 'low', 'concentrations.', 'Thus,', 'it', 'is', 'highly', 'plausible', 'that', 'the', 'reported', 'associations', 'between', 'mortality/morbidity', 'with', 'daily', 'PM', 'con-', 'centrations', 'merely', 'reflect', 'other', 'causal', 'relationships', 'that', 'underlie', 'both', 'daily', 'PM', 'changes', 'and', 'daily', 'fluctuations', 'in', 'mortality', 'and', 'morbidity.', 'REFERENCES', 'Aldous,', 'M.', 'B.,', 'Holberg,', 'C.', 'J.,', 'Wright,', 'A.', 'L.,', 'Martinez,', 'F.', 'D.,', 'Taussig,', 'L.', 'M.,', 'and', 'Group', 'Health', 'Medical', 'Associates.', '1996.', 'Evaporative', 'cooling', 'and', 'other', 'home', 'factors', 'and', 'lower', 'respiratory', 'tract', 'illness', 'during', 'the', 'first', 'year', 'of', 'life.', 'Am.', '/.', 'Epidemial.', '143:423-430,', 'i', '#', '516008280']