of air pollution changes. Given that the effect of ozone on mortality independent of particulates is still being debated, we re-evaluated the effect of ozone restricting the analysis to those studies that controlled for particles in the statistical analysis.

# _Properties of PM, 0 and Ozone_

Aerosol air pollutants (molecular aggregations) have been shown to be more toxic than gases. This is because gaseous compounds are eliminated by the respiratory system much more easily than aerosols, which are rapidly deposited or absorbed. (Wilson and Spengler 1996).

_PM1 O_ In the field of air pollution epidemiology, there is now much more interest in the study of PMlo and PM2.5 particles, and the organic and inorganic compounds in them (Wilson and Spengler 1996, Pooley and Mille 1999). The particles produce toxic effects according to their chemical and physical properties. Their effects on susceptible individuals are much more severe than those produced in normal individuals (Schlesinger 1995, Wilson and Spengler 1996).

The extent of particle penetration into the respiratory system is determined by particle size. Only particles less than 10 p.m in diameter enter the respiratory system. This is the reason for focusing on PM1o (particles less than about 10 [im).<sup>9</sup> Once particles have entered the respiratory tract, depending on their size, they can accumulate in different sites. Evidence suggests that many of the health effects associated with PM1o can be attributed to even smaller particles (Pope and Dockery 1999, Ghio and Samet 1999). Since, however, most epidemiological information refers to PM 10, and for the ZMVM there is little air quality information on smaller particles, we restricted our analysis to PM1O.

The chemistry of suspended particles complicates empirical epidemiology enormously and has not been analyzed in detail yet, so little epidemiological evidence is available on the influence of the chemical composition of particles. Since little is known about the chemistry of PM1o found in Mexico City, this aspect is not accounted for. This contributes to the uncertainties associated with exposure-response modelling.

_Ozone._ Ozone is a poorly soluble but highly reactive gas. "Bad" ozone (as opposed to "good" ozone in the stratosphere) is mainly produced in the troposphere (ground level) by a series of sunlight-driven reactions involving oxides of nitrogen and volatile organic compounds. Inhaled ozone is partially depleted in the upper airways but a major fraction reaches the lower airways. In the body ozone can react with uric acid, which is secreted by human submucosal airway glands and is present in near millimolar/liter (mmolI1) concentrations of nasal surface liquid. Pryor and his colleagues have proposed that some of the toxic products of the latter reaction (hydroxyhydroperoxides, hydroxyaldehides) are important mediators of ozone effects on underlying epithelium. Bromberg (1999) has calculated that ozone per se does not even reach the epithelial cell apical membrane in conducting airways.

> 9 Actually, the metric is not the size, but the aeolic behavior of particles, as measured in equipment that mimics the human respiratory system. This metric comes close to size.

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