In the wake of news about China's "airpocalypse," a new study highlights the necessity of environmental regulations in reducing acid rain pollution caused by uncontrolled growth in rapidly growing East Asian megacities.

The study, conducted by Purdue University researchers, showed that clean-air regulations markedly cut down acid rain in the United States, Europe, Japan, and South Korea in the last three decades, but that acid rain has increased drastically in rapidly growing East Asian megacities that lack effective anti-pollution policies.

Acid rain is caused by "wet deposition" of nitrates and sulfates, which are produced by fossil fuel combustion in cars, other vehicles, and factories. The pollutants rise into the atmosphere and form acidic compounds with water and oxygen, then rain down as precipitation that can cause major ecological damage, leading to the acidification of lakes, rivers, and soil, and speeding up the decay of buildings and infrastructure. The pollutants that cause acid rain can also damage human health, leading to lung disorders like asthma and bronchitis.

The Clean Air Act in the United States started policing air pollution in the 1970s, and according to study author Suresh Rao, regulations there and in Europe, Japan, and South Korea in recent decades have dramatically reduced acid rain levels.

Most of the world's new megacities (cities with over 10 million residents), however, are mushrooming throughout other parts of Asia, many of which currently have nonexistent or ineffective air pollution regulations. According to Forbes, in 1950 New York City and Tokyo were the world's only two megacities. By 2025, 21 of the world's 37 megacities will be in Asia.

The Purdue team analyzed publicly accessible data about acid rain near several American and East Asian cities from 1980 to 2010, in order to compare the effects of environmental regulations by region during that time period. The results were published in the May issue of the journal Atmospheric Environment.

"Our analysis of wet deposition (acid rain) data provides compelling evidence that clean-air policies and enforcement of environmental regulations are profoundly important," said Rao in a news release.

The results show that annual rates of acid rain are uniformly low across major American cities, which the researchers say is a result of effective environmental regulations, enforcement, and engineering solutions that limit emissions in the United States.

Quickly-growing cities in East Asia, on the other hand, have high levels of acid rain that match their unregulated air pollution. Most dramatically, rainwater nitrate and sulfate concentration in the northwestern Chinese city of Xi'an was 10 times higher than in New York City.

"This is the same thing that transpired in the United States in the period leading up to the 1970s," said Rao. "We had rapid urban growth, rising emissions and rising wet deposition, which is analogous to what's happening now in places like Beijing and New Delhi."

"This is encouraging," said coauthor Heather Gall in the news release. She suggests that since the United States dramatically reduced acid rain in the past three decades, a period generally marked by economic and population growth, East Asian megacities can do the same by adopting and enforcing effective environmental regulations without sacrificing growth.

"When mitigation strategies are widely adapted, it is possible for cost-effective engineering solutions to protect the environment while simultaneously allowing people to maintain the same quality of life."

Still, the United States already had significant infrastructure by the 1970s, when growing environmental alarms led to effective regulation. Many East Asian megacities are unprecedentedly enormous and growing far faster than any in Europe or the United States did in the 20th century.

It's unclear how effectively these megacities, many of which are in China, will be able enforce environmental regulations in coming decades, though the need to do is becoming increasingly dire. Recent research indicated that 1.2 million people in China die as a result of air pollution each year, out of 3.2 million pollution-related deaths worldwide.