In today’s Washington Post, Anne-Marie O’Connor reports on the remarkable progress Mexico City has made in clearing its air. In 1992, the UN declared it to be the most polluted in the world – but today the city of nearly 20 million people doesn’t even crack the Top Ten list of the most polluted. O’Connor explains the transformation:

In 1992, the United Nations declared Mexico City the most polluted on the planet. High ozone levels were thought to cause 1,000 deaths and 35,000 hospitalizations a year. Thermal inversions held a toxic blanket of dirty air over a grimy city that seemed to embody the apocalyptic “Makesicko City” of the fiction of Mexican author Carlos Fuentes.

Mexico was forced to act. It replaced the city’s soot-belching old cars, removed lead from gasoline, embraced natural gas, expanded public transportation, and relocated refineries and factories.

Change was gradual, but the pace has quickened in recent years.

The presence of lead in the air has dropped by 90 percent since 1990. Suspended particles — pieces of dust, soot or chemicals that lodge in lungs and cause asthma, emphysema or cancer — have been cut 70 percent. Carbon monoxide and other pollutants also have been drastically reduced.

“It is no longer an emergency situation,” said Raul Estrada, a spokesman for Greenpeace in Mexico, “though obviously, it is not 100 percent satisfactory.”

Ozone is still a problem, with levels exceeding international standards for 530 hours during the last year. The city’s atmospheric monitoring system has just begun measuring additional chemicals, and its new information may help officials tackle this problem.

O’Connor emphasizes the lessons that Mexico City can hold for megacities in developing countries like China and India. As I read the article, though, I couldn’t help thinking about the lessons it holds for my own city.

Improved transportation is a key elements of Mexico City’s pollution-prevention system, and it’s constantly facing new challenges – O’Connor cites increased suburbanization and a doubling of car ownership, which now stands at 4.2 million. Mexico City will likely have to expand its impressive public transportation network to ensure that these increases don’t return the region to its former conditions.

Like agencies in many other regions, the DC area’s transit agency is facing a severe budget shortfall. It has proposed a combination of fare hikes and service cuts to address the problem, but these changes are likely to reduce ridership and could send the system into a death spiral. The Fair Share for Metro campaign is urging the governments of DC, Maryland, and Virginia to increase their funding in order to avert service cuts; so far, only Virginia has made any move towards doing so. The Washington Post urges the jurisdictions to cough up the cash, noting that the transit system is essential to the region’s functioning and prosperity.

The article on Mexico City reminds us that public transportation is also essential to the region’s health. The more trips people take by public transit rather than by car, the less congestion and air pollution the city will experience. Even those who aren’t riding the bus or the train benefit from the clearer roads and cleaner air. Our cities should invest in public transportation, because it’s an investment in public health.