The smog got thicker on Friday and blotted out the sun through the day. It was described by one expert as a "toxic cocktail of poisonous gases". Not coincidentally, Friday's air pollution levels were actually worse than the air quality recorded on Diwali last year (see graphic).

In 2001, alarmed by Delhi's rapidly deteriorating air quality, the judiciary had ordered the conversion of all public transport vehicles to the cleaner CNG. It can now be officially said - based on government data on nitrogen oxide and particulate matter (see graphic) - that gains in air quality made due to the CNG switch have now been squandered away.

"This is a clear case of haze and smog due to high pollution levels," said R K Jenamani, director incharge of IGI Met.

Fog screen over city: Blame it on high pollution levels

Since October 28, when smoggy conditions descended over the city, Delhi has already recorded 91 hours of below 1,000 m visibility. The weatherman said similar conditions would prevail till Sunday.

"The city has been having fog in the morning because of very high level of moisture as a remnant of cyclone Nilam. There is also no wind. However, what develops later in the day cannot be fog since the temperatures by then are too high."

Delhi's current PM10 and NO2 levels are the worst ever. On Diwali last year, PM10 touched a high of 597 mg/cu m while on Friday, it ranged between 448 mg/cu m and 1,440 mg/cu m. Similarly, NO2, the highest level of which during last Diwali was 133 mg/cu m, ranged between 47 mg/cu m and 394 mg/cu m on Friday.

Says a fact sheet by the Centre for Science and Environment, "(Delhi's) rising nitrogen dioxide (NO2) levels signal a looming problem. This pollutant strongly correlates with motorization and is held responsible for a range of respiratory ailments and sudden death syndrome in infants and so on. PM10 levels are unacceptably high and continue to be substantially above the national ambient air quality standards. After initial stabilization, particulate levels have begun to rise again".

Anumita Roychowdhury, associate director at CSE says the rise in PM10 and NO2 levels can be directly attributed to the growing number of vehicles on the city roads, since Delhi has added no other source of pollution. "The gains from switching over to CNG have been frittered away completely. Over the past few years, we have been recording steadily rising levels of pollutants like PM10, PM2.5, oxides of nitrogen and ozone. All of these, except ozone, come from vehicular emissions," she said.

Delhi adds more than 1,000 vehicles to its roads daily. The total number of registered vehicles currently is over 65 lakh. "Its effects are visible in our air right now, which is nothing more than a toxic cocktail of poisonous gases," said Roychowdhury.

In a paper published in 2007, IGI Met director in-charge Jenamani argued that rising pollution levels in the city had led to a greater number of fog and smog hours over the years as pollutants like PM and aerosols act as "additional sources of cloud condensation nuclei necessary for fog formation". The paper said alarming rise in fog and pollution was causing a fall in maximum temperature over Delhi.

Smog is the name given to a mixture of fog and particulate matter, or smoke, arising out of polluting. A rise in pollution levels during winter months is a common phenomenon. This occurs as cold air stays close to the ground, giving pollutants smaller area for dispersal. Absence of wind, as is the case these days, also aids in higher pollution levels as the pollutants remain suspended in air. However, the early onset of smog this time is also because levels of particulate matter have scaled new heights, crossing 1,000 mg/cu m at some locations against the standard 100.

Met officials say that the low visibility conditions will remain till at least November 4 and even then, will dissipate quite slowly. "A fresh western disturbance is coming around November 5-6 which will result in west/northwesterly winds over northern India. By then, the moisture left behind by the cyclone would have dissipated. Things should start clearing up by then," said a Met official.

The last time such low visibility conditions prevailed over Delhi in November was in 2008 when a spate of three cyclones in the month led to 245 hours of below 1,000m visibility. "Normally when cyclones come towards the earlier part of winter, they lead to smog and not fog as temperatures are still not quite low. We are not expecting dense fog for another couple of weeks at least," said Jenamani.

Asthma cases on the rise

The elderly and children are bearing the brunt of the smoggy weather, say doctors, as cases of asthma and chronic bronchitis rise sharply. "We are seeing many patients who are not able to walk at home due to breathing difficulty," said Dr Aroop Basu of Gangaram Hospital. The gloomy skies and fall in temperatures are also leading to a surge in headaches, mood swings and depression.