NEW DELHI: Air quality in Delhi deteriorated sharply on Friday, marking an ironic start to restrictions on car usage in the city. Scientists said this was in line with research which shows that many factors other than the use of personal vehicles determine the level of air pollution.The data also highlights the importance of the speed and direction of wind, which can bring pollutants, particularly dust from Rajasthan and fumes from other towns, apart from blowing away particulates hovering over the metropolis, scientists said.As soon as the odd-even scheme, which aims to cut pollution by debarring half the cars from roads on the basis of the last digit of their registration numbers, was launched for the second time in the city, the intensity of very small particulate matter, call PM-2.5, jumped to "poor" levels in the morning.PM-2.5 levels deteriorated to the "very poor" category in the afternoon, official data from the Indian Institute of Tropical Meteorology showed.The level of PM-2.5 had remained "good" and at times on the lower side of "moderate" level in the first two weeks of this month, when there was no restriction on car usage. These micro particles, one-thirtieth the width of a human hair, are small enough to penetrate human lungs and enter the circulation system, and are considered extremely dangerous.The density of PM-2.5 rose to 120 micrograms per cubic metres from below 80 a day earlier. A scientist involved in monitoring air quality said slowing down of the breeze over Delhi may have contributed to the rise in pollution.A study by scientists of the Indian Institute of Technology had earlier shown that cars accounted for barely a third of the particulates in the air, with other contributors such as trucks and dust contributing much more to pollution.Pollution monitoring stations of the Indian Institute of Tropical Meteorology showed that six of the ten locations in Delhi and its immediate neighbourhood had a "very poor" score in PM-2.5 density while the remaining four were relatively better off with a "poor" rating.The Delhi government had notified the second run of odd-even scheme from April 15-30, saying this was necessary due to the "alarming" level of vehicular pollution.It said in a notification on April 11 that the government is "satisfied that further steps are required to control vehicular pollution caused by non-transport four-wheeled vehicles (motor cars, etc)", and "hereby orders in the interest of public safety…prohibitory/restrictive measures shall be in vogue in the area of National Capital Territory of Delhi".