1991); in Denver between 30 and 45 percent (Chestnut and Rowe 1988)<sup>and in California 61 percent</sup> (Kleinman et al. 1989).9 The two parameters needed to apply equation (1) to the Mexican<sup>conditions are</sup> baseline RRAD, which is 3 (Secretaria de Salud 1991, and personal information),<sup>and the change in</sup> the annual concentration of FP. If the purpose is to achieve the<sup>legislation standard, we have to</sup> substitute (& Annual FP) for the difference between the current average<sup>levels of FP (119 ,ug/m3)</sup> and the legislation standard (50 jg/m3). Substitution of these values<sup>in equation (1) leads to a</sup> change in RRAD of 2.4. This is almost the baseline value of RRAD (3<sup>in Mexico), suggesting that</sup> most RRAD would disappear if FP were controlled in Mexico<sup>City.'0</sup> The population in the metropolitan area of Mexico City is not precisely known. We used the<sup>round estimate of 17 million</sup> in 1990, of which _55_ percent were adults."' In estimating the monetary value<sup>of saving 2.4 restricted</sup> activity days per person per year, we assumed that half the cases of RRAD<sup>implied a work loss day</sup> (this is based on the fact that the elasticity of WLD to pollution was<sup>roughly half of that for</sup> RAD).'<sup>2</sup> The total number of work loss days saved by bringing the current levels of<sup>FP pollution</sup> to the legislation standards is thus 11.2 million (17 million x 0.55 x<sup>0.50 x 2.4).</sup> We took the average wage in industry in Mexico City-- US$4 per hour- to<sup>value each</sup> day lost. Excluding all suffering and eventual medication to treat minor<sup>RRAD, the morbidity</sup> benefits associated with the reduction of the current levels of PP pollution<sup>to the legislation standard</sup> are estimated at US$358 million.

_9. If the data available is concentration of total suspended particles (ISP - the_<sup>_most common_</sup> _measure of particulate matter pollution) rather than FP, the same equation_<sup>_should be used, with the_</sup> _right hand term being multiplied by the percentage of FP in TSP; in the case_<sup>_of Mexico, this fraction_</sup> _would be from 0.4 to 0.65._ _10. If instead of attaining legislation levels the purpose were to eliminate_<sup>_completely FP(f_</sup> _possible), the reduction in RRAD would be greaterthan 3, the current_<sup>_levels of RRAD. This may_</sup> _strengthen the argument that there are indeed threshold levels below which_<sup>_further reduction in the_</sup> _concentration of FP leads to no extra health benefits._ _11. Demographic estimates were based on Sistema Nacional de Salud (1988)_<sup>_and on the_</sup> _Encuesta Nacional de Salud (Secretaria de Salud 1988)._ _12. An alternative approach would be to use the data presented in the Encuesta_<sup>_Nacional de_</sup> _Salud, which indicates that 76 percent of the cases of acute diseases did_<sup>_not imply WI)D (diasde_</sup> _incapacidad), while the remaining 24 percent implied one or more days. The_<sup>_expected number of_</sup> _dias de incapacidad was 1.13, meaning that on average all cases of RAD_<sup>_implied a WLD (in fact_</sup> _1.1 WID); we used the more conservative approach._ 12