COMPARISON OF OUTDOOR AND CLASSROOM OZONE EXPOSURES FOR SCHOOL-CHILDREN IN MEXICO-CITY

To evaluate methods of reducing exposure of school children in southwe st Mexico City to ambient ozone, outdoor ozone levels were compared to indoor levels under three distinct classroom conditions: windows/door s open, air cleaner off; windows/doors closed, air cleaner off; window s/doors closed, air cleaner on. Repeated two-minute average measuremen ts of ozone were made within five minutes of each other inside and out side of six different school classrooms while children were in the roo m. Outdoor ozone two-minute average levels varied between 64 and 361 p pb; mean outdoor levels were above 160 ppb for each of the three condi tions. Adjusting for outdoor relative humidity, for a mean outdoor ozo ne concentration of 170 ppb, the mean predicted indoor ozone concentra tions were 125.3 (+/- 5.7) ppb with windows/doors open; 35.4 (+/- 4.6) ppb with windows/doors closed, air cleaner off; and 28.9 (+/- 4.3) pp b with windows/doors closed, air cleaner on. The mean predicted ratios of indoor to outdoor ozone concentrations were 0.71 (+/- 0.03) with w indows/doors open; 0.18 (+/- 0.02) ppb with windows/doors closed, air cleaner off; and 0.15 (+/- 0.02) ppb with windows/doors closed, air cl eaner on. As outdoor ozone concentrations increased, indoor ozone conc entrations increased more rapidly with windows and doors open than wit h windows and doors closed. Ozone exposure in Mexican schools may be s ignificantly reduced, and can usually be kept below the World Health O rganization (WHO) guideline of 80 ppb, by closing windows and doors ev en when ambient ozone levels reach 300 ppb or more.