A recent study done by Rossanna Bossi, a senior research scientist at Denmark’s Department of Atmospheric Environment, has found that the level of benzo[a]pyrene, one of the most toxic polycyclic aromatic hydrocarbon (PAH) known to mankind, in Kathmandu’s air is more than three times higher than levels recommended by the European Union. This was disclosed today at a seminar organized by the Environment Sector Programme Support (ESPS) project of Ministry of Population and Environment.

In November, 2003, Bossi had taken two samples from five different monitoring stations in Kathmandu and analysed them in Denmark. Based on the results, Bossi said that, "annual mean concentration of benzo[a]pyrene in Kathmandu is expected to be at least three times higher than EU recommended level."

PAH is a group of highly toxic and carcinogenic compounds, which primarily results from incomplete combustion of fossil fuel. Among PAH compounds, benzo[a]pyrene is commonly used as an indicator as its toxicity is one of the highest and it accounts for about 75 percent of carcinogenicity of a PAH mixture.

The EU recommends that the concentration of benzo[a]pyrene in the air should be no higher than 1 nanograms per cubic meter (ng/m3). However, samples taken from Patan Hospital, Putali Sadak, Thamel and Bhaktapur in November 2003 found the concentration of benzo[a]pyrene to be 2.32, 3.16, 3.23, and 4.3 ng/m3 respectively. The only place where the concentration was below the EU recommended level was in Macche Gaon, which is a village located in the south-eastern part of Kathmandu Valley, about 150 meters above the valley floor.

The high level of PAH in Bhaktapur is probably due to the transport of Kathmandu’s PAH towards Bhaktapur due to westerly winds. Because benzo[a]pyrene attaches itself to fine particles that are emitted from incomplete combustion, they are easily transported by the wind and they enter the body when the particles are inhaled.

Bossi also said that diesel vehicles and two stroke engines are probably the main sources of PAH in Kathmandu.

Bossi had also measured the PAH levels on September 18, 2003, which was a Nepal Bandh day when there were very few vehicles on the streets. She found that the PAH level on the Bandh day was about one fifth of the level recorded during a normal week-day in November. This clearly indicates that polluting vehicles are the main source of toxic chemicals in Kathmandu’s air.