Air pollution in Beijing reached the second-highest alert level for the first time in more than a year, and air quality was labeled "hazardous" at 10 a.m. on Sunday morning. The Beijing municipal government raised the alert level after thick smog clouds settled over the city, which are not expected to blow over until Tuesday, according to the Associated Press. Until then, the Ministry of Environmental Protection is advising residents - especially children and the elderly - to stay indoors. The Ministry is also sending out teams to make sure that nearby factories are complying with environmental regulations.

The levels of PM2.5 - the smallest and most deadly form of air pollution particles - in the air in Beijing at 10 a.m. on Saturday morning was 300 micrograms per cubic meter, according to Bloomberg News. The World Health Organization considers the safe level of this carcinogen to be 25 micrograms per cubic meter or less.

As of 10 a.m. on Monday in Beijing, those levels rose to 480 micrograms, according to the World Air Quality Index based in Beijing, China.

The alert level was raised on the same day that Environmental Minister Chen Jining claimed that China had already met the environmental targets for pollution reduction outlined in their 12th five-year plan six months ahead of schedule, according to Chinese new agency Xinhua. However, Chen did claim that China would only see substantial environmental improvements if it is able to further reduce emissions by another 30 to 50 percent. China is currently the largest producer of carbon emissions in the world.

Part of the sudden increase in pollution levels could be due to the onset of colder weather, causing residents to rely on pollution-heavy methods for heating their homes, suggested the South China Morning Post. This happened in the city of Shenyang earlier this month, where the levels of PM2.5 reached over 1,200 micrograms per cubic meter.