Incidences of air pollution are not an isolated event but are a continuous process as the sources of pollution operate throughout the year. Air pollution is a great concern to humanity at large as its consequences have transcended national boundaries and have become a threat on a global scale.

The level of pollution may be found higher during the day when most of the economic activities are taking place and when most of the people are moving around in different pursuits. The level also varies according to the time of the year due to two reasons; the first one linked with weather conditions and the second one involves certain types of economic activities that take place seasonally.

According to world health statistics (2016); about 4.3 million people die every year due to air pollution caused by cooking fuels; 3 million people die due to outdoor pollution. Air pollution is one of the major environmental hazards that the Bangladesh is experiencing currently. Bangladesh is rated to be an extremely vulnerable one with an overall score of EVI (environmental Vulnerable Index) - 340. World Bank has said, reducing air pollution could save 3,500 lives from death and prevent 230 million cases of respiratory disease annually in Bangladesh.

Yale University, USA (2016) has published a report on environmental performance index (EPI). The position of SAARC countries are - Bangladesh (174), India (141), Pakistan (144), Srilanka (108), Nepal (149), Bhutan (110), Maldives (137), Afghanistan (176); out of 180 countries. In south Asia, the position of India and Pakistan is better than Bangladesh. Even many African countries are way in advance from us in spite of their low per capita income and higher incidences of extreme and hardcore poverty.

About 98% of cities in the low and middle income countries, like Bangladesh, consisting more than 100,000 populations violate the WHO air quality guidelines. Dhaka is assumed to be one of those cities. Regarding urban air pollutants, the annual mean concentration of fine particulate matter (PM 2.5), 2014; in Bangladesh-89.7, India- 73.6, Afganistan-64.1, Nepal-75.7, Bhutan-39.0 and Srilanka-28.6.

In Bangladesh, the level of air pollution is highest in Dhaka followed by Chittagong and Khulna, the two other industrial cities. The concentration of CO, NOx, PM10 and CO2 in Mohakhali is respectively, 2519 μg/m3, 376 μg/m3, 547 μg/m3 and 435 ppm; Farmgate- 7730 μg/m3, 752 μg/m3, 289.92 μg/m3, 590 ppm; Science lab- 5726 μg/m3, 113 μg/m3, 169.93 μg/m3, 500 ppm. Mogbazar- 5726 μg/m3, 339 μg/m3, 383.53 μg/m3, 475 ppm. In Chittagong, the concentration of PM2.5 was 307 in 2012. Estimation of 2017 mentions, it is now 371.

The concentrations of sulphur dioxide, nitrous oxide and carbon dioxide in the air have been increasing over the years. In winter, smog becomes a part of the daily life of the city dwellers. Department of environment has mentioned the sources of air pollution in urban area - 60% pollutants from brick kilns; 18% from road dust, soil dust; 10% from vehicles smog; 10% from burning of biomass; 10% from construction activities.

Construction of unplanned buildings, roads and earth excavation for land development lead to soil erosion. Increased demand for bricks is the outcome of an expanding construction industry, which follows rapid urbanization. To meet this demand, the number of brickfields keeps on increasing in the outskirts of urban areas. Brickfields are also responsible for air pollution in the area. These emit large amounts of smoke, fly ash, SO2 and dust which are carried by the wind in all directions. As these are situated conveniently near the cities, they pollute both the urban and the rural environment.

Increase in urban population has created a tremendous pressure on the existing network of intra-urban and inter-urban roads and highways as both the number and the volume of vehicular of traffic increases within the city or in its outskirts at specific time of the day, especially during rush hours. The number of motorized vehicles plying in streets of Dhaka was around 140,000 in 1995; 185,000 in 2000 and 290,000 in 2005. Now it is around 3, 85,000. Apart from that, there is a question of registration and non-registration. Motorized vehicles are escalating day by day. Two stroke three wheelers, diesel powered trucks, buses and defective motor vehicles of all types are responsible for the emission of NOx, SOx, CO, particulates and hydrocarbons.

Another contributing factor is the fuel being used by the automobiles spread − CO, CO2, NOx, SO2. The petroleum refinery of Bangladesh supplies diesel which contains 1% sulphur per litre although the international standard is 0.2%. The metal industries spread hydrogen sulphide, sulphur, fluorides, metal fumes( lead, zinc), smoke, particulates; power plants spread - fly ash, CO, sulphur and nitrogen oxides; trains spread - lead, smoke, hydrocarbons, aldehydes and peroxides.

Roadside digging by different agencies providing utilities services seem to be taking place all year round and lacks coordination. When the sewerage lines are in place, the soil is removed and the road is repaired, a new set of digging is begun perhaps by the telephone company or the roads and highways authorities. There is no zoning in most of the urban areas. Industries are set up in residential areas with total disregard for the safety of the inhabitants of the area. In Dhaka itself, in the older section, iron smelting, welding, match factories, factories producing plastic products, chemical industries and tanneries all coexist with residences of people.

Inhalation of carbon dioxide is injurious to health. It reduces the oxygen carrying capacity of the blood. Haemoglobin, a constituent of the blood is a complex protein that carries oxygen to body cells. The oxygen attaches itself to the haemoglobin forming oxyhaemoglobin. Carbon monoxide combines with haemoglobin more than 200 times faster than oxygen. It forms carboxyhaemoglobin and reduces the oxygen carrying capacity of the blood. Consequently, cells are deprived of oxygen. Exposure to 1-5 ppm of SO2 for 1 hour may be considered a threat to human health. It is a pulmonary irritant. It has definitely been shown to aggravate respiratory problems. The gas may also have carcinogenic effects. Larger particulates may lodge in the nasal passage and are easily dislodged. The smaller particles easily penetrate into the lungs and lodge there causing severe breathing problem. As particulate matters consist of chemical substances, these lead to different types of diseases such as, black lung disease suffered by coal miners, pulmonary fibrosis of the asbestos workers and emphysema suffered by urban dwellers. Some particulates are carcinogenic in nature and may cause cancer in humans. Nitrogen dioxide irritates the respiratory tracts - may cause pulmonary fibrosis and emphysema when concentration level is elevated. Aldehydes irritate the eyes and respiratory tract.

Air pollution is a major environmental risk to health. By reducing air pollution levels, countries can reduce the burden of disease from stroke, heart disease, lung cancer and both chronic and acute respiratory diseases particuarly asthma. The lower the levels of air pollution, the better the cardiovascular and respiratory health of the population will be, both long and short term. Apart from that, indoor smoke is a serious health risk for some 3 billion people who cook and heat their homes with biomass fuels and coal. Outdoor air pollution in both cities and rural was estimated to cause 3.7 million premature deaths worldwide in 2012. If we don't minimize the air pollution then we would lose a significant portion of our labor force due to health impacts.

Much of the air pollution may be reduced if we address the following areas: As fuels used by motor vehicles and industries contain chemicals that pollute the air - sulphur in coal and diesel, tetraethyl lead in gasoline, fuel cleaning is necessary. We have to use low pollutant fuels; wherever possible, compressed natural gas should be used.

In Bangladesh, smoke and CO emission may be reduced by 80% if the use of CNG is ensured; catalytic converters may be installed in automobiles to reduce emission of NOx, CO and hydrocarbons.

The import of two stroke three wheelers must be stopped. A rigorous monitoring system must be developed to detect faulty vehicles plying the roads and violators should be penalized. Emission from industries must be controlled using better, more efficient technology of emission control. Economic policies such as setting emission standards, emission charges and polluters to pay may be applied to reduce emission by industries. Brick kilns should use clean fuels and locate away from human settlements.

Shishir Reza Environmental Analyst and Associate member of Bangladesh Economic Association