The Indian capital declared a pollution emergency on Thursday as toxic smog hung over the city for a third day and air quality worsened by the hour. Illegal crop burning in the farm states surrounding New Delhi, vehicle exhaust emissions in a city with limited public transport and swirling construction dust have caused the crisis, which arises every year. The problem has been compounded this year by still conditions, the weather office said. A U.S. embassy measure of tiny particulate matter PM 2.5 showed a reading of 608 at 10 a.m. when the safe limit is 50. An hour before it was 591. PM 2.5 is particulate matter about 30 times finer than a human hair. The particles can be inhaled deep into the lungs, causing heart attacks, strokes, lung cancer and respiratory diseases. Residents complained of headaches, coughs and smarting eyes. "Waking up with a headache, breathlessness and throat irritation every day," Bhavani Giddu wrote on Twitter. Many people stayed home and restaurants in some of the city's most crowded parts were deserted. "I'd like to assure people that the central government shall do everything possible to bring about improvement in air quality in Delhi and the Nation Capital Region," federal environment minister Harsh Vardhan said as authorities faced criticism for failing to take steps to fight a problem that erupts every year. The haze covered India Gate, a war memorial in the centre of the city where Britain's Prince Charles and his wife Camilla paid their respects on Thursday. The city will curb car use next week, the state government said, the latest attempt to clean the air. New Delhi will follow an "odd-even" scheme for five days starting Monday in which cars will be allowed on the roads based on whether their number plates are odd or even. "It is an emergency situation," said Delhi Transport Minister Kailash Gahlot. In other measures, commercial trucks have been banned from the city unless they are carrying essential commodities, all construction has been stopped and car parking charges raised four times to force residents to use public transport. Schools have been shut for the week. But experts said these measures were unlikely to bring immediate relief. "There is such a cloud over us that you probably need artificial rain or some such to clear this," said Dr Vivek Nangia, a pulmonologist at Delhi's Fortis hospital. Video images shot by ANI, a Reuters affiliate, showed farmers illegally burning crop stubble in Rohtak, about 65 km from Delhi. Farmers in Haryana, where Rohtak is located, and Punjab, the two big agrarian states surrounding Delhi, burn millions of tonnes of crop waste around October every year before sowing the winter crop of wheat. State authorities say it is hard to enforce the ban unless farmers, a powerful political constituency, are given funds to buy machinery to clear their land. Punjab Chief Minister Amarinder Singh said in a Twitter post: "Situation is serious but Punjab helpless as problem is widespread and state has no money to compensate farmers for stubble management."