The Relationship of Daily Mortality to
Suspended Particulates in Santa Clara
County, 1980-1986

by David Fairley*

"For the San Francisco Bay Area, this risk (from low levels of fine particulate pollution) is much greater than the risks from any toxic identified so far." "The entire excess death rate in the San Francisco Bay Area occurs during the wood burning months (Fairley 1990). The Relationship of Daily Mortality to Suspended Particulates."


This paper explores the relationship between daily mortality and suspended particulates in Santa Clara County, CA, for years 1980 to 1986. An association was found between high particulate concentrations and increased mortality. This association persists after adjustment for temperature, relative humidity, year, and seasonally. Contrary to expectation, the magnitude of the particulate effect appears the same or larger than that estimated for London, despite Santa Clara County's cleaner air. The persistence of an effect at these lower particulate concentrations suggests that the particulate variable may be acting as a surrogate for some constituent particles, such as acid aerosols.

From the concluding discussion in the paper (bold type added)

This possibility has regulatory consequences. For example (for argument's sake only) suppose that nitrates were the real culprit. Then regulations which reduced auto traffic or which mandated cleaner-burning engines would be beneficial, while regulations restricting construction dust would not. This example highlights a possible danger imposing particulate regulations on total particulates.

Having stated the caveats, it is also important to stress that the finding of any health effect indicates that there may be a sizable health risk from particulates. If the mortality signal were not strong enough, no statistically significant effect would be observed. By way of comparison, although temperature certainly affects mortality, the apparent effect of temperature was not statistically significant in many of the models fit. While it is misleading to try to quantify the risk exactly, it can be said with confidence that the effect is on the order of at least one in a thousand early deaths due to particulate concentrations in excess of the State standard. For the San Francisco Bay Area, this risk is much greater than the risks from any toxic identified so far.

This study suggests that particulates may be a health risk at concentration lower than previously suspected. Further study is indicated to attempt to identify which constituents (if any) within the particle mix are causing the health problems. Speciated particulate data could be collected and compared with mortality in a multiple regression. Perhaps other measures such as emergency room visits for respiratory problems or absences from school might be a more sensitive dependent variable than mortality. Similar studies in other areas would help to clarify whether or not particulates are the culprit.


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