Motorised vehicles emit 80 per cent of the particulate matters in the air of the capital, revealed a recent Bangladesh Atomic Energy Commission (BAEC) study.

The study, conducted between April 2000 and February 2001, found the average emission level in Dhaka to be 89mg per cubic metre, whereas in developed countries like the United States it is a mere 15 mg/m3.

According to the BAEC study, air in the capital is thick with fine particulates that mainly come from smoke emitted by the auto-

rickshaws, buses and trucks. The most harmful among these particulate are carbon monoxide, sulphur oxide and nitrogen oxide.

On February 18 a sample from

high speed diesel (HSD) showed that PM 2.5 mass was 262.26. On February 25 and 26, it came down to 52.54 and 40.33 respectively. Incidentally, those were hartal days and less than five per cent of the city's mechanised vehicles took to the streets.

The PM10 count too displayed significant fall over the same period. It was 217.72 on February 18, and 44.82 and 42.59 on February 25 and 26 respectively.

PM 2.5 and PM 10 remain suspended in the air for hours and even days, and can spread far from the source. These particles enter the respiratory tract, reaching deep into lungs. PM10 includes all particles likely to pass through the nose and mouth. PM2.5 consists of particles able to reach the inner parts of the Respiratory tract, especially the alveolar regions.

Medical experts warn that these may cause a number of ailments like heart disease, impaired vision, manual dexterity and learning disability. In extreme cases, these poisonous particles may lead individuals to death.

Studies showed that ultra-fine particles (with diameters of less than 0.05 microns) get cleared from the lungs in a very slow process and can penetrate the pulmonary interstitium, where they cause inflammation.

The largest source of fine particulate formation is the incomplete combustion of fossil fuels and biomass. Low fuel quality, inefficient combustion process, and poor vehicles and equipment maintenance all contribute to particulate emission.