MEXICO CITY—Two men and two women rode topless down Mexico City's main boulevard Wednesday to protest the health effects of the city's ongoing pollution problem and encourage residents to use bicycles instead of cars.

The cyclists had painted their bodies with the outlines of diseased lungs and other internal organs they say are affected by car-caused pollution.

The protest was sponsored by children's and bicycle rights groups.

The city has made progress since the mid-1990s, when air quality was unacceptable almost every day of the year.

Now the city has unacceptable amounts of ozone or other contaminants on about half the days. While the amounts of ozone, carbon monoxide and other pollutants such as lead have declined dramatically, the city hasn't made much headway against suspended particles since the mid-2000s.