are made about the nature of these errors. The validity of these assumptions needs to be ascertained.

Until the effects of measurement error are understood and taken into account, the association between exposures to ambient particulate matter (or its size fractions and biologically significant chemical constituents) and health effects cannot be estimated without acknowledging the source of uncertainty and its potential effect on risk estimates or particulate-matter reduction strategies.

Data need to be collected to ascertain the nature of measurement error. Once that is understood, the effects can be ascertained and methods can be applied to correct for error.

Much of the data required to address the issue of exposure measurement error will be collected through personal exposure studies conducted as part of the enhanced exposure monitoring component of the overall particulate research program. Nonetheless, additional studies designed to characterize distribution measurement error distributions will be needed. Such studies will involve replicate measurements under the same conditions, and are estimated to cost approximately $1.0 million annually for the first, second, third, and fourth years. Methodological development of exposure measurement error methods is recommended on an overlapping time frame, beginning with a workshop to identify methodological approaches ($100,000 in year 1), and followed by a five year program of methodological development and application, costing $500,000 per year.