If you live in Dhaka, a city that is perennially drowned in a sea of polluted air, you may think that a scarlet sunrise or sunset blazing across the horizon is a sight to behold. However, there is an ugly story behind this pretty picture. It is air pollution. Indeed, pollutants of any kind in the air will make sunrise and sunset colourful.

Pollutants in the air arise from two major sources: natural and anthropogenic. Globally, the largest sources are natural events: dust storms, forest fires, volcanoes, earthquakes, biological decay and the like. In sheer quantity, natural pollutants often outweigh the anthropogenic pollutants that generally create the most significant long-term threat to the biosphere. Why? Natural pollutants come from widely dispersed sources or infrequent events. As such, they do not substantially raise the ambient pollutant concentration, and thus have little effect on the environment.

Some of the major anthropogenic pollutants in an urban setting are effluents from vehicles, emissions from industries and power plants using fossil fuels. They emit large quantities of harmful pollutants—carbon monoxide, carbon dioxide, sulphur oxides, nitrogen oxides, particulates, hydrocarbons and photochemical oxidants—in restricted areas, making a significant contribution to local air pollution levels. Other sources of pollution are municipal and agricultural waste sites, brick fields, foundries, metal smelters and waste incineration facilities. Refineries, which emit several pollutants, also make a huge impact on the quality of air. All these pollutants are precursors to the formation of smog, a term coined to describe a mixture of smoke and fog. It is the worst form of air pollution.

Smog is produced through a complex set of photochemical reactions involving the above-mentioned pollutants. They react in the presence of sunlight to produce a witch's brew of virulent chemicals. Among some of the worst are formaldehyde, peroxyacyl nitrate (PAN) and acrolein. Furthermore, ozone is formed at the ground-level through chemical reactions involving unburned hydrocarbons in gasoline, volatile organic compounds, various oxides of nitrogen and sunlight. Problematic ozone levels occur often on hot summer afternoons when there is little wind and temperatures soar above 30 degrees Celsius. The net result is a brownish orange shroud called photochemical smog, occurring more frequently in large cities with high rise buildings where there is less air circulation and more accumulation of pollutants in the lower atmosphere.

To make a bad situation worse, smog remains under siege for days if it is accompanied by temperature inversion, a phenomenon where air temperature increases with altitude instead of decreasing, resulting in a warm-air lid over cooler air anywhere from ground level up to few thousands of feet into the atmosphere. In an area experiencing inversion, the warm-air lid prevents ground-level air from rising. Consequently, pollutants in the cool, stable and quiescent ground air become trapped below the warm layer of air, creating dirty air with dangerous concentrations of noxious pollutants.

The pollutants in the air do not respect international borders and are carried by wind to faraway places. Hence, anthropogenic air pollution is a global environmental problem instead of regional or local, continuous rather than episodic.

A measure of outdoor air pollution is the Air Quality Index, or AQI, a yardstick that runs from 0 to 500. It rates air conditions across a city/country based on concentrations of five major pollutants: ground-level ozone, particulate matter, carbon monoxide, sulphur dioxide and nitrogen dioxide. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI of 50 represents good air quality with little potential to affect public health. When AQI is above 100, air quality is considered to be unhealthy, at first for certain sensitive groups of people—sick, elderly and children, and then for everyone as AQI gets higher. If the AQI is greater than 200, the air is considered hazardous for the entire population.

Dhaka has the dubious distinction of being one of the 10 most polluted cities in the world, with AQI invariably close to 200. Arguably, these cities are often labelled as "hell with the lid off." According to The Health Effects Institutes' State of Global Air Survey, the entire population of Bangladesh has been consistently exposed to unhealthy levels of pollutants in the air since 1990.

Over the past few decades, researchers have unearthed a wide array of health effects which are caused by exposure to air pollution, particularly smog and ozone. Among them are respiratory diseases—asthma, emphysema, coughing, shortness of breath, changes in lung function and lung cancer. Children are at a greater risk of damage to lungs because their respiratory systems are still in the developmental stage. Cardiovascular diseases, immune system impairment, adverse pregnancy outcomes such as preterm birth and significant decrease in life expectancy are other health-related effects of air pollution.

Human body has very little defence against the injurious effects—burning sensations in the eyes, nose, and throat, irritation of the skin and nasal passages—of the three pernicious constituents of smog—formaldehyde, PANs and acrolein.

Most fuel contain some toxic mineral contaminants, such as lead and mercury, both highly potent neurotoxin. These nonburnable contaminants may be carried off by hot combustion gases, escaping into the air as particulates. Exposure to these deadly particulates have serious health-related consequences too, primarily neurological disorder, severe disability and muscular tremor.

The effects of human-caused air pollution are not limited to people. Acid rain, for example, is formed when emissions of sulphur dioxide and nitrous oxide combine with moisture in the air to create sulfuric and nitric acidic precipitation, thereby acidifying lakes with detrimental effects on aquatic biome. It can also cause structural damage to buildings and monuments, especially those made of limestone or marble, as well as destroy plants and crops.

Indoor pollution can be worse than outdoors pollution in some cases. Air pollution inside buildings is accelerated by the toxicity of materials like asbestos, radon, pesticides and tobacco smoke, mildew, mould, mites, dust and pet dander, together with poor ventilation and humidity. Appliances that produce combustion fumes, especially cooking stoves and water heaters, emit carbon monoxide.

Most indoor pollutants, except carbon monoxide, asbestos and radon, are responsible for irritating but non-lethal allergic reactions. Prolonged exposure to air with high levels of carbon monoxide could be lethal, while radon and asbestos can cause lung cancer.

While there is currently no proven link between air pollution and Coronavirus (COVID-19) mortality, one peer-reviewed study into the 2003 SARS outbreak showed that patients in regions with moderate air pollution levels were 80-85 percent more likely to die than those in regions with low air pollution. COVID-19 is similar to SARS and can cause respiratory failure in severe cases.

Satellite images from NASA show a surprising effect of COVID-19 outbreak in China: reduction in air pollution. A "significant decrease" in pollution over Wuhan and the rest of China is attributed in part to an "economic slowdown" resulting from the virus outbreak.

An unexpected consequence of air pollution could be cooling the climate by offsetting some of the global warming that has occurred so far. That is because certain aerosols—sulphate, for instance—can reflect part of the sunlight back into space before it reaches the Earth's surface. Call it unwittingly geoengineering the climate. Nevertheless, even if pollutants reduce global warming, it is not desirable to have them in our lungs.

Finally, because of the vastness of the atmosphere, we felt that it could absorb any conceivable amount of abuse by us. We have, therefore, used the air, and in turn our lungs, as a receptacle for hundreds of noxious pollutants. But with clean air technologies, targeted regulations, effective laws and strict emission standards, it is still possible to go far enough back in time to a period when the air was relatively pure.

Quamrul Haider is a Professor of Physics at Fordham University, New York.