Delhi's transport minister Gopal Rai issued an appeal on Saturday. “I urge parents to either carpool or utilise their bikes to drop and pick up children from school," he said, proposing his solution for Monday's odd-even blues.Ironically, this can prove to be detrimental for the city's fight against pollution. An emerging trend in recent days has been a shift to two-wheelers which are also an unsafe mode of transport and cause more fatalities than cars.According to Delhi transport department, there are 56,81,265 two-wheelers -scooters and motorcyles -in the city. The figure is significant since it was 10 lakh less three years ago -46,44,146 in 2012-13. This accounts for 64% of the total vehicles in Delhi, and the number is going up. The number of private four-wheelers at present is 27,90,566, accounting for 32.51% of the total registered vehicles in the city. This too was far less in 2012-13 -just over 23 lakh. Though the transport department is not aware of how many two-wheelers were added in the first phase of odd-even and now, officials admitted that the number has been going up steadily. More than 3 lakh two-wheelers were registered in 2015 and it is expected to go up this year. “The number of two-wheelers registered every year is almost double of fourwheelers," said an official.While two-wheelers run on petrol -considered far cleaner than diesel, which fuels several cars -their emissions are not so innocuous. An IIT-Kanpur study had found that two-wheelers account for 33% of particulate matter (PM) emissions in the transport sector, second only to trucks. Two-wheelers should not be exempted from the odd even scheme. Their pollution impact is high be cause of their sheer numbers,“ says Anumita Roy Chowdhury of the Centre for Science and Environment Chowdhury of the Centre for Science and Environment (CSE). “The exemption and propagation of their use needs to be rethought. Though norms allow them to emit more NOx than a car now, it will equalise with petrol cars at Bharat Stage VI level in 2020. So, control their numbers by making their usage more expensive than using a bus.As two-wheelers are very fuel efficient, their operating cost per km is cheaper than the minimum bus fare. This will have to be corrected with taxes and bus fare policy," said Chowdhury .Two-wheelers are most space and fuel efficient.Their large numbers, however, make their PM emission share higher, but it is so negligible per vehicle that it is not regulated under the Bharat Stage norms.According to an analysis by International Council on Clean Transportation (ICCT), two-wheeler emissions include particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HC) and sulphur oxides (SOx), which vary with the fuel's sulphur content.PM emissions from two-wheelers are less toxic and lower compared to diesel emissions, experts say.The chemical nature of PM emissions from bikes is very different from that of diesel, which is why bikes are considered a lesser evil, CSE researchers said.IIT Kanpur's draft study also revealed that 60% to 90% of PM 2.5 emissions in certain parts of the city are from diesel vehicles.What also worries experts is the safety aspect. “They (two-wheeler users) are at the highest risk of fatal accidents, particularly children riding without helmets. But two-wheelers have smaller engines and are far more efficient than cars. So they are in no way as polluting as cars,“ said Geetam Tiwari, TRIPP (Transporation Research and Injury Prevention Programme) chair profes sor at IIT Delhi.“A significant increase in motorized two-wheeler flow can have the effect of increase in deaths and serious injuries as these vehicles are much more hazardous for their occupants than all the other vehicle modes,“ said a recent analysis of odd-even phase I by IIT Delhi.