Government agencies on Thursday heaved a sigh of relief with WHO's latest urban air quality data showing a slight improvement in Delhi's ranking among the "most polluted" city's of the world. Civil society organisations, however, took it as an opportunity to discuss how much more needed to be done in Delhi to meet both the national and WHO safe standards of 10 micrograms per cubic metres. Meanwhile, experts said the introduction of BS IV norm in 2010 and an increase in the number of vehicles with this fuel standard by 2014 could be linked to an improvement in the air quality. Broadly, emission from BS IV vehicles is 40% less than that from the BS III-compliant ones. The Delhi government released a statement, saying the WHO data indicated a "definitive positive" trend. "There is no significant drop in air pollution levels but a slight decline from 2014 onwards. We are not sure what the reasons are, but it's good news that Delhi is not the most polluted city any more," said a DPCC scientist. According to the WHO report, New Delhi is the 11th most-polluted city in the world. Central Pollution Control Board scientists, however, pointed out that the methodology for arriving at the annual mean for PM 2.5 might be flawed. "It's not a good idea to extrapolate PM 2.5 from PM 10 because the share of fine particles varies from that of PM 10 by 18-98%, which is a huge range. The data is informative but we can draw conclusions," said Dipankar Saha, the head of Air Lab at CPCB. Green think thank Centre for Science and Environment (CSE) said that "the annual average PM 2.5 levels in Delhi have reduced by 20% since 2013". But pollution levels in several other Indian cities have worsened, including Patna, Allahabad, Gwalior and Kanpur. Said Anumita Roychowdhury, CSE's executive director: "This indicates air pollution is now a national crisis and needs strict and aggressive action across all cities of India." One of the reasons for improvement in Delhi's pollution levels could be more vehicles meeting the BS IV standard. "The beginning of the second phase of action in Delhi has stabilised the air pollution trends. A much larger number of vehicles are meeting the BS IV standard that was introduced for new vehicles in 2010," a CSE statement added. More interventions have been made in the recent years with the latest ones being the Supreme Court's directive imposing environment compensation charge (ECC) on heavy commercial vehicles entering Delhi and the restriction on entry of pre-2006 trucks. These measures have halved the number of trucks on Delhi roads, which otherwise contribute to about 30% of the transport sector pollution. Apart from this, the Rajghat thermal power plant was shut down last year. The one at Badarpur is operating at 30% of its capacity, CSE said, adding that much more needed to be done on an urgent basis. Greenpeace India said the WHO report was a grim reminder of severe pollution levels in the city. "We are glad to see that the government has taken some steps to reduce air pollution: such as the new emission standards prescribed for thermal power plants and introducing BS VI standard for vehicles. The critical thing now is to ensure that these are implemented as soon as possible to reduce the public health crisis due to air pollution."