The capital city with its slums and polluting industries is plagued by all sorts of environmental degradation, says a report. Continuous migration of people into Delhi, whose population has already crossed the nine million mark, "has strained the existing infrastructure within the city to the point of no return", says the report by A. K. Lal, Planning Engineer at the Environmental Planning division of Town and Country Planning Organization here. The 1,100 slum clusters, 94,000 industrial units, over 22 lakh vehicles and 4,400 metric tonnes of municipal solid wastes have thrown the city's environmental health out of gear, Lal reported in the Indian Journal of Environmental Protection. The most serious problem is air pollution, with Delhi ranked fourth among 41 polluted cities monitored worldwide. An estimated 1,280 tonnes of pollutants are emitted by 22 lakh vehicles, and vehicular pollution account for more than two-thirds of Delhi's total air pollution. Almost two lakh vehicles are added to Delhi each year, two-thirds of them being two-wheelers, Lal's report says. The vehicles are major sources of nitrogen oxides, carbon monoxide and hydro-carbons. The next major contributor to the city's air pollution are the industries with the Central Pollution Control Board (CPCB) having already identified five major and 22,000 significantly polluting industries in the capital. The prime sources of pollution include thermal power plants, brick kilns, potteries, steel rolling mills and induction furnaces. Data collected between 1988 and 1993 indicates that the suspended particulate matter (SPM) usually exceeds permissible limits, the report says. Studies from 1987 to 1993 also show that particulate lead levels have exceeded that limit prescribed by the World Health Organization (WHO) at some busy intersections in the capital. Air pollution, caused especially by emission of poisonous gases such as nitrogen oxides, carbon monoxide and lead oxide, has been linked to lung cancer, asthma and bronchitis. in fact, four out of every five cancers are linked to toxic and hazardous chemicals in the environment. Nitrogen oxides emitted by vehicles are respiratory irritants that cause breathing problems, while sulphur dioxide and suspended particulate matter (SPM) damage lung function. SPM also affects larynx, brain, liver, kidneys and the stomach, increasing lead pollution from industries and automobiles can cause cancer. on inhalation, lead can be absorbed in the brain, liver, kidneys and blood leading to brain damage, muscle paralysis, convulsions and even death. Delhi is facing formidable problems of water pollution and sewage disposal too. The Yamuna picks up nearly 1,800 million liters daily (MLD) of domestic and industrial waste waters, while only three quarters of the city are covered by sewage facilities. Majority of Delhi's unauthorized and resettlement colonies and squatter settlements do not have a sewage system, while about 40 percent of Delhi's sewage is discharged in to the Yamuna without treatment, the report says. The city is also ridden with the noise pollution caused by automobiles, railways, aircraft, industrial machines, public address systems and social and religious activities. A recent study by CPCB has shown that noise levels in several industrial and commercial areas were higher than the stipulated levels. However, the most disturbing trend was in silence zones where noise levels exceeded permissible limits not only during day but also night.