Air pollution levels in India’s capital have soared to hazardous levels this week, leaving a toxic grey haze hanging over the city and causing poor visibility. The pollution has been so pervasive that national monuments were largely obscured by thick smog. Many who venture outside suffer teary eyes and troubling coughs.

Delhi was already considered one of the world’s most polluted cities, and it’s only gotten worse this month. Air quality deteriorated so significantly that the local government declared a public health emergency, schools were shut down, and flights were cancelled. By one estimate, breathing Delhi’s air for one day has the health impacts of smoking at least 25 cigarettes. Although Delhi typically experiences marked increases in air pollution around November, this year’s pollution prompted Delhi’s Chief Minister, Arvind Kejriwal, to call the city a “gas chamber.”

A ‘perfect storm’

Delhi typically experiences “striking spikes in air pollution” in the late-fall early-winter time period, although the deterioration of air quality this year is particularly concerning, Vijay Limaye, climate change and health science fellow at the Natural Resources Defense Council who has been studying air quality in India, tells TIME.

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