Levels of tiny particles polluting the air in Hong Kong's Central business district are 104 per cent higher than the World Health Organisation safety standard during evening rush hour, according to a new study that links traffic jams with higher pollution. While the amount of the pollutant PM2.5 emitted by private vehicles is relatively lower than by buses, it accounts for more than 40 per cent of all particulates from vehicles on Des Voeux Road Central during the busiest periods. Private cars clogging up traffic in turn caused higher levels of PM2.5 to be emitted from buses, the study found. Using PM2.5 readings taken from monitoring devices fitted onto trams, the year-long data shows readings of PM2.5 reaching 51ug/m3 from 7pm to 8pm - which is more than twice the limit set by the WHO at 25ug/m3. Watch: Friends of the Earth video about Hong Kong's air pollution "Even though the data is collected on trams, the air quality affects everyone in that area; not just tram-riders, but also those on the streets and especially those working there," said environmental affairs officer Adrien Chan Kam-cheuk from Friends of the Earth, which conducted the study. The higher readings during the rush-hour period from 6pm to 8pm were made worse by traffic congestion - of which private vehicles were the main cause, said Chan. Private vehicles block bus stops and occupy tram tracks, slowing traffic. Due to the nature of bus engines, slower travelling speed increases tailpipe emissions, which leads to the worsening of roadside air quality. Between 6pm and 7pm, an average of 298 private vehicles, 160 buses, 136 taxis and 31 trams use Des Voeux Road Central. Friends of the Earth is calling for public transport to be given priority in using roads. "Even though buses emit more pollutants, they are more efficient and perform a public service," said Chan. In contrast, private vehicles are inefficient because they have limited carrying capacity while taking up road space. There are only 0.4 passengers in each private vehicle or taxi for each unit of road length occupied, compared with 3.9 passengers on buses and trams. The concern group's assistant environmental affairs manager, Melonie Chau Yuet-cheung, called on the government to encourage public transport and minimise traffic congestion. "It is ironic that zero-emission transportation like trams ends up suffering from pollutants from other vehicles," said Chau. "It's also discouraging - they may consider putting in air conditioning in order to make it better for the passengers. Apart form supporting the MTR, the government should show support to other more environmentally friendly transportation." For busy arteries such as Des Voeux Road Central, the environmentalists want more bus-only lanes and signalling priority at junctions to allow buses and trams to go first. The study was conducted from March 2014 to February this year, in collaboration with the University of Science and Technology, measuring PM2.5 readings on Des Voeux Road Central along the stretch from Western Market to Chater Garden. Connaught Road Central, Des Voeux Road Central and Queen's Road Central are the major thoroughfares in the area, and all three were already extremely busy, the Transport Department said in a written response to the study. "Traffic arrangements must be able to fulfil the needs of different users. Current transport arrangements are balanced and fulfil different users' needs." Eastbound Des Voeux Road Central from Pedder Street to Chater Garden is already a bus lane, and the department said it made appropriate adjustments to traffic lights to ensure buses did not get stuck. A previous study by Civic Exchange and HKUST found that Hong Kong's air pollution levels exceeded health safety standards for 280 days in the past year, with Des Voeux Road Central and Hennessy Road in Wan Chai registering the worst readings.