This week the smog in Delhi, India, reached a seasonal high of 50 times the safe limit set by the World Health Organization. Authorities closed New Delhi’s schools, limited the amount of trucks on the road, and shut down construction work as residents reported headaches and sore throats. Air pollution in the region usually rises in winter because of the burning of crops in agricultural areas—the lower temperatures cause smoke to stay in the air as it blows into cities and joins forces with urban pollutants.

A health emergency went into effect throughout Pakistan’s Punjab province on November 15 as a heavy swath of smog continued to linger in the region. All schools and institutions were closed and a lockdown was imposed in Pakistan’s second largest city, Lahore—which has a population of more than 11 million—as well as Multan. The smog, as in India, was partly due to crop burning, but auto emissions and pollution caused by construction and industrial work were also main culprits.

The term smog is derived from the words smoke and fog, but not all smog is the same. There are two distinct types: sulfurous and photochemical. Sulfurous smog, also called “London smog,” results from a high concentration of sulfur oxides in the air and is caused by the use of sulfur-bearing fossil fuels, particularly coal. Photochemical smog, also known as “Los Angeles smog,” occurs most prominently in urban areas that have large numbers of automobiles—and it requires neither smoke nor fog. It occurs when nitrogen oxides and hydrocarbon vapor emissions undergo photochemical reactions in the lower atmosphere. The highly toxic gas ozone arises from this reaction, and some nitrogen dioxide is produced from the reaction of nitrogen oxide with sunlight.