In winter, carbon from vehicular emissions and dust combine with moisture -- which has lately been as high as 80 per cent -- to form a thick smog. Unlike in the summer when convection currents cause the pollutants to dissipate, condensation makes the smog settle low over the city, taking a toll on the city's health. These particulates irritate lung tissue, further aggravating respiratory ailments like asthma and chronic bronchitis and causing acute infections of the throat.

With Delhi's abysmal pollution record -- it is the fourth most polluted city in the world, with 2,000 tonnes of pollutants spewed into its air every day, according to S.M. Sarin, acting director, Central Road Research Institute -- there is reason enough to worry. A World Health Organisation study has found significant correlation between high levels of sulphur dioxide, a major constituent of diesel fumes, and the prevalence of dysponea (breathing difficulty) and chronic cough.

Doing as much damage are the oxides of nitrogen, six times higher than the permissible limit in many areas, that decrease gaseous exchange in the blood, thus reducing blood function. Furthermore, sulphur dioxide and nitrogen oxides damage the respiratory mucous membrane, which increases the absorption of pollutants into the bloodstream, besides making the person more prone to allergies. It's no wonder then that the incidence of respiratory ailments is 13 times higher in Delhi than in other parts of India.

Even if a healthy person is exposed to polluted air for a short while, he may experience some non-specific symptoms, such as uneasiness in the chest, cough, breathlessness, fatigue and listlessness. ``If the exposure to pollutants is temporary, these symptoms go away in a few hours after the person has been taken away from the polluted area,'' says Dr J.C. Suri, chest physician and Head, Department of Pulmonary and Critical Care Medicine, Safdarjang Hospital.

What is more worrying is the effect of pollutants on those who suffer from hypersensitivity disorders like asthma. ``Though there are no substantive studies available showing pollution actually causes asthma, there is enough evidence to suggest that it aggravates the sub-clinical cases -- potential candidates for the disease, in other words, have a higher chance of having an attack in a polluted environment,'' points out Suri. Agrees V Ramalingaswamy, professor national research, All India Institute of Medical Sciences: ``Poor environmental quality is directly responsible for up to 25 per cent of all preventable illnesses and constitute as much as one-third of all preventable illnesses in children.''

Cough is often the chief symptom of asthma, especially in children, but this problem is very likely to go unrecognised because pollution can also irritate the throat and cause a dry cough.

Though pollution is just one of the combination of causes that can lead to asthma, it is significant that incidence of the disorder among children in the Capital has shot up drastically. Says Iqbal Malik, director, Vatavaran: ``In 1988, asthma in school children was one in 25, but now it has gone up to one in five in 1998.'' She also found that 60 per cent of these students have no family history of the disease.

``One person dies every 15 minutes from air pollution in the 36 cities on which data is available on particulate levels,'' says Anil Agarwal, director, Centre for Science and Environment. ``If data about ozone and benzene levels was available, the figure would be even higher.'' If the Bhopal gas tragedy killed 2,500 people in 1984, air pollution kills 10,000 people in Delhi every year. This number, however, can come down if those suffering from bronchial allergies take simple precautions like staying indoors on smoggy days, avoiding congested roads and keeping an inhaler handy in the crucial winter months.