While world leaders met this week in Paris to try to solve the climate crisis, another environmental calamity was underway in China.

Beijing choked on its worst smog of the year on Dec. 1st with particulate matter measured at extremely dangerous levels. Many of the Chinese capital's iconic landmarks disappeared behind the thick brown miasma, including Tiananmen Square and Beijing National Stadium. Residents scrambled to stay indoors to escape from the noxious haze.

China's Ministry of Environmental Protection blamed the toxic conditions on "unfavorable weather" with stagnant air keeping pollution from coal-fired power plants inside the city.

"What happens is if a city is surrounded by mountains, the cars (and factories) spew out the stuff, and the smog sits on top of the city," explained Norman Edelman, Senior Scientific Advisor at the American Lung Association, to weather.com. "If you have a stagnant air mass, the pollution just builds up and builds up."

Beijing is geographically situated between mountain ranges, which creates an inversion layer , or cold air held in place by a warmer air mass.

The air quality became so bad that the government issued an orange alert on Sunday, which requires industrial plants and construction sites to either cut back or shut down work in attempt to stem the impact of the dangerous air.

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/air-quality-index-chart.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/air-quality-index-chart.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/air-quality-index-chart.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > The Environmental Protection Agency deems air quality levels between 301 and 500 micrograms per cubic meter as “hazardous,” Beijing's readings were measured at 611. (nashville.gov) (nashville.gov) The U.S. Embassy in Beijing measured the air quality index at 611 micrograms per cubic meter; for context, the World Health Organization deems 25 migrograms per cubic meter to be safe and between 301 and 500 to be hazardous.

(MORE: 14 Ways Air Pollution Hurts Your Health )

"Smog is very bad for your lungs," Edelman noted. "It can cause flare ups like chronic lung disease, COPD (chronic obstructive pulmonary disease), chronic bronchitis and asthma."

Early in the week, Beijing residents crowded hospitals seeking help from medical conditions brought on by the smog.

"These are things we don't see anymore in the United States," Edelman noted, saying that solutions to the problem rest with the government. "The Clean Air Act has been quite successful in reducing air pollution here. It's still dangerous, and we lose hundreds of people to air pollution each year, but it's not anywhere where it used to be."

To illustrate the severity of the impurities in the air, a Beijing performance artist used a vacuum cleaner to suck up the dust from the smog to create a brick of "condensed pollution." The artist told the New York Times , "Dust represents the side effects of humankind’s development, including smog and building-site dust ... When I first arrived in Beijing, I wore a hygienic mask for a few days, but later I stopped. In smog like this, there’s no escaping.”

A spokesperson for the Chinese embassy in the United States defended the country's environmental record in a statement to weather.com, saying the smog was one of the drawbacks of its growing economy.

"The smog is a reflection of the broader issue of climate change and environment deterioration that requires a comprehensive strategy," the spokesperson said. "In the past few decades, China has seen rapid economic growth and significant improvement in people's lives. However, this has taken a toll on the environment and resources. Having learned the lesson, China is vigorously making ecological endeavors to promote green and low-carbon growth. The Chinese government is

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