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Mexico City's adopting a popular method to fight air pollution. It probably won't work.

Mexico City is often covered in a hazy layer of smog.
Mexico City is often covered in a hazy layer of smog.
Susana Gonzalez/Getty Images

The 20 million people that live in Mexico City and its surrounding suburbs are plagued daily by a hazy layer of smog.

But after years of struggling to maintain healthy air quality levels, the Mexican capital says it has a temporary fix: Keep more cars off the streets.

Starting Tuesday, Mexico City's metropolitan authorities will implement a comprehensive "no circulation" policy to keep all privately owned cars off the road one day per week, and one additional Saturday per month, the Associated Press reported.

They plan is to keep the rule in effect until late June — the start of the region's rainy season, which typically helps improve air quality. Municipal authorities also said they will be lowering the threshold for declaring smog alerts.

This isn't new: Mexico City has had "no-circulation" laws in place since the late 1980s, which limited certain cars from the roads depending on their license plate numbers (for example, on Mondays cars with license plate numbers ending in "5" or "6" weren't allowed on the road). But those attempts, and many others across the world, haven't worked. So there's plenty of concern about whether this new policy will suffer from the same problems.

Why ordering cars off the road doesn't always work

Lucas Davis, an energy researcher at UC Berkeley, has studied Mexico City's past attempts to order cars off the road. Surprisingly, he found that past programs have actually led to more air pollution in the long term.

One reason, he said, is that complying with these policies is a "HUGE hassle." Mexico City authorities hope people will take public transportation — the city has a metro and dedicated bus lanes. But public transit can be "slow and inconvenient," Davis said. What's more, riding a bus has a (perhaps undeserved) reputation as uncomfortable or dangerous.

So, instead, people find other ways to get into cars. Some single-car families decide to invest in a second car with a different license plate number. Others take taxis or Uber or Lyft. That helps explain why pollution doesn't seem to go down. "I just think that once people become drivers in Mexico City they don't go back," Davis said.

Instead, these rules are mostly a costly inconvenience. "A rough calculation suggest these costs amount to over $300 million per year, or $130 per vehicle owner," according to Davis's research.

Why does Mexico City continue to rely on these "no circulation" days?

So if it's so ineffective, why do authorities persist? "The Mexico City politicians want action," Davis said. "They see a problem and politicians want to do something. These driving restriction make them feel like they are doing something."

To be fair, the latest law has tried to close some of the loopholes in earlier rules. Previous "no circulation" rules had exemptions for vehicles that passed smog-checks. But many residents would just pay mechanics to certify their cars as "lower-emissions" in order to stay on the road. This new rule will not allow for any exemptions.

Municipal authorities in Mexico City said they are planning to implement more modern technology at smog-check centers starting July 1. But eventually, Davis said the city will have to work harder to close loopholes and possibly consider more modern technology, citing cities like Singapore where residents are taxed for driving on major roads during peak hour.

"It could be done — we are there technologically, we are just not there politically," Davis said.

There are also legal barriers. A law that kept cars that were more than eight years old from being on the road one day a week was overturned by the Supreme Court last year, a decision that many environmental activists now point to as cause for recent elevated pollution levels.

Mexico City's air quality is dangerous

Mexico City declared emergency ozone levels two weeks ago after experiencing the highest levels of air pollution since the 1980s and 1990s. Currently, areas of the city still have air quality deemed "unhealthy" by the World Air Quality Index.

In the World Air Quality map below, reflecting air quality measures on Thursday, red and orange tags show areas with "unhealthy" levels of air pollution, which can cause adverse health affects:

It has been well documented that these pollutants have adverse effects on people's health, from watery eyes to respiratory irritation. Conditions escalate depending on residents' ages and current health statuses, confirmed by a post-mortem study of young people's hearts conducted in Mexico City.

Studies conducted in 2010 on the costs of air pollution showed that reducing the levels of pollutants like ozone and PM10 by just 10 percent could save upward of $760 million a year — meaning it would eliminate more than 33,000 emergency room visits and more than 4,000 hospital admittances due to respiratory issues and reduce infant deaths by the hundreds.

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