Mexico City’s air was cleaner last year than at any previous time in the 1990s as a result of stricter enforcement of pollution laws, according to a government study. Authorities took emergency measures like keeping kids inside during recess only five days last year, compared with 37 days in 1998 and 62 days in 1990, the study found. A separate government report found that the Mexican state oil monopoly, Pemex, is the nation’s worst air polluter. Profepa, a government environmental watchdog established in 1992, has been reviewing the environmental performance of some 6,000 companies in the country.