New Delhi declared a pollution emergency and banned the entry of trucks and construction activity on Thursday. Toxic smog covered the city for a third day, and air quality worsened by the hour. The crisis in the Indian capital was caused by various factors: illegal crop burning in the surrounding farm states, vehicle exhaust emissions in a city with limited public transport and swirling construction dust, Reuters reported. The problem has been compounded this year by still conditions, according to the weather office. Residents complained of headaches, coughs and stinging eyes, and many have stayed home.