The Context

History has witnessed numerous disasters, both natural and manmade, that are remembered even today for their devastating impact on human health and environment. Every disaster / accident leaves behind it, victims, either in form of direct loss of lives and properties or indirectly by altering the natural processes. Atmospheric pollution has often been the cause of many accidents, which had far reaching effects on the environment. Winters often provide favourable atmospheric conditions for the pollution episodes to occur. In this article, I have tried to bring forward the major air pollution episodes and their environmental impacts, which I hope, will help the readers realize the importance of clean environment.

The Major Air Pollution Episodes

From centuries, urban air pollution has posed a problem for city dwellers. The earlier manifestation of pollution was the smoke produced by burning poor quality coal in relatively cold climates in Northern Europe. This caused high sulphur dioxide (SO2) and particle exposure, sometimes with serious health impacts. The Industrial Revolution introduced point sources of larger emissions from various processes. The most important of these was the burning of coal for energy production, resulting in SO2 and combustion particles (smoke).

Coal burning in small and large sources was allowed to continue without required abatement well into the 20th century. Elevated stacks were the only mode of pollution abatement. Large emissions of SO2 and smoke particles overloaded the atmosphere during temperature inversions in stable high-pressure situations with stagnant air, leading to notorious air pollution episodes. The 1952 London episode was the worst, resulting in an estimated 4,000 deaths in five days.

Although the components of air pollution have changed over the years, with the emergence of industrial sources like toxic gases - methyl isocyanide (CH3CN) in the case of Bhopal disaster in 1984; coal still remains an important source of air pollution. In addition, vehicular pollution sources have expanded rapidly in the last three decades. Traffic activity in urban areas of industrialised western nations has witnessed a four to five fold increase over the decades. In developing nations, this increase has been ten-fold to more. Prime examples of urban air pollution are the smog in London and photochemical smog in Los Angeles and Mexico City. The table below shows the history of major air pollution episodes and their effects. A brief description of these major pollution episodes and its effects are presented.

London Episodes, 1873-1963

The industrial revolution in the 19th century saw the set in of air pollution in Europe on a large scale. The industries and the households relied heavily on coal for heating and cooking. Due to burning of coal for heat during the winter months, emissions of smoke and sulphur dioxide were much greater in winters than they were during the summer months. Smoke particles trapped in the fog gave it a yellow/black colour and this smog often settled over cities for many days.

The effects of smog on human health were evident, particularly when smog persisted for several days. Many people suffered respiratory problems and increased deaths were recorded, notably those relating to bronchial causes. The smog-related deaths were first recorded in London in 1873, when it killed 500 people. In 1880, the toll was more than 1000. London had one of its worst experiences of smog in December 1892. It lasted for three days and resulted in about 1000 deaths. Despite gradual improvements in air quality during the 20th century, eight air pollution episodes occurred in London between 1948 and 1962. The December 1952 episode is the major episode in the history of air pollution.

The “Killer Smog” began on Thursday, Dec. 4, 1952 as a high-pressure air mass created a subsidence temperature inversion over southern England. With the particulate and SO2 levels going up due to extensive use of coal as fuel for space heating and electric production, the fog turned black. At the same time the high-pressure area stalled and became stationary. The build up of pollutants combined with the fog resulted in essentially zero visibility. Within a matter of three days, the pollutants were concentrated enough to cause deaths. The old and respiratory affected died first, but younger people exposed to the outside atmosphere were also affected. The maximum daily SO2 concentration recorded at that time was 1.34 ppm (about 4000 µg/m3, standard SO2 conc. in clean dry atmosphere is 0.0002 ppm) and smoke levels were 4.46 mg/m3. The Great London Smog lasted for five days and lifted on 9th Dec, resulting in about 4000 deaths.

Bhopal Disaster, 1984

In the mid night of 2nd - 3rd December 1984, in a densely populated area of Bhopal, Central India, a poisonous vapor burst from the tall stacks of the Union Carbide pesticide plant. About forty tons of toxic gases had leaked from the Carbides Bhopal plant and spread throughout the city. The cause was the contamination of Methyl Isocyanate (MIC) storage tank with water carrying catalytic material.

Residents of the city awoke to clouds of suffocating gas, unaware of the magnitude of the devastation, which had engulfed them. The city of Bhopal was immediately turned into a city of dead bodies, and the whole place smelled of burning chilli peppers. Of the million people living in Bhopal at that time, more than 2,000 died immediately (one fourth of actual figures) and as many as 300,000 were injured. In addition, about 7,000 animals were affected, of which about 1000 were killed. The precise number of deaths still remains a mystery till date. The degree of injury was so high that about 30% of the injured were unable to return to their jobs. Among the survivors, most of them still suffer agonizing pain from the disastrous effects of the massive poisoning while there are still apprehensions of the future generations being affected. The Bhopal Disaster was the worst episode in the history of industrial air pollution.

Donora Fog, 1948

Horror visited the US Steel company town of Donora on the Halloween night of 1948, when a temperature inversion descended on the town. Fluoride emissions from the Donora Zinc Works smelting operation and other sources containing sulphur, carbon monoxide and heavy metal dusts were trapped by weather conditions, causing 20 deaths within 14 hours.

Cold ground and high-pressure conditions intensified the elevated inversion of the anticyclone that arrived in the region. The situation was aggravated by local conditions of meteorology, industrial pollutant emissions and peculiar terrain of the area. The meteorological conditions and the geographical characteristics of the area produced a strong temperature inversion with a temperature gradient as high as 33oC/km. The fog was held close to the ground by the stability of the elevated inversion layer. During the third and fourth days of the episode, as ambient levels of pollutants escalated, almost half of the population of 14,000 people became ill. Almost 43 % of the population in Donora and Webster, PA experienced the effects of the smog. Most of the affected were above the age group of 60 years and above (29% of this group were seriously affected). The health effects were mainly symptoms affecting the lung, and in particular, upper respiratory symptoms such as nasal discharge, constriction of the throat, or sore throat were experienced.

Meuse Valley, 1930

An episode occurred in the first week of December 1930, when a thick mist lay over large parts of Belgium. On December 3rd, 4th and 5th, several thousand cases of acute pulmonary attacks occurred in the densely populated valley of the Meuse, east of Liege, resulting in 60 deaths. Post episode investigations led to the conclusion that the cause was poisonous products in the waste gas of the many factories in the valley, in conjunction with unusual climatic conditions. During that time, the day temperature was a little above freezing point while at night it measured up to 10oC below, while the wind speed was only 1-3km/hr. It was impossible to indicate any definite substance or chemical compound as the cause, but the investigators were of the opinion that the disaster in all probability had been brought about by sulphur dioxide (SO2) or oxidation products of that compound, of which quantities were found in the factory smoke. The investigation also looked into the question of fluorine intoxication, however its role is doubtful.

The cases of illness were reported after the mist had lasted about 2 days. It was estimated that the total number of cases was several thousands. In three days, there were 60 deaths, of which fifty-six were in the eastern half of the valley and only four deaths were reported from the west of Engis. The area around Engis was the worst affected.

Mexico, Poza Rica, 1950

A catastrophic exposure episode involving the release of large quantities of hydrogen sulfide occurred in Poza Rica, Mexico in November 1950. Poza Rica, a city of 22,000 people located about 210 km northeast of Mexico City, was then the centre of Mexicos leading oil-producing district and the site of several oil field installations, including a sulphur-recovery plant. An early morning malfunction of the waste gas flare resulted in the release of large quantities of unburned hydrogen sulfide into the atmosphere. The unburned gas, aided by a low-level temperature inversion and light early morning breezes, was carried to the residential area adjacent to the plant area. Residents of the area succumbed while attempting to leave the area and assisting stricken neighbours. Within a matter of 3 hours, 320 persons were hospitalised and 22 were killed.

Aftermath of Disaster

As mentioned earlier in this critique, every disaster leaves victims behind them. Human beings are the ultimate sufferers of these disasters either in the form of human lives or change in environmental quality.

Across the UK, episodes of extremely poor urban air quality occurred less frequent from around 1900, as industry and residential areas moved away from the centre of cities. A trend towards using town gas and electricity also contributed, although the use of coal in their production, inevitably, involved emissions. However, pollution episodes such as the Great Smog still occurred not only in London, but elsewhere too.

The health impacts of London fog were irritation of the glands in the bronchial tubes that produce mucus, leading to over production of mucus. People slowly choked on this mucus succumbing either due to lack of oxygen (asphyxia) or heart failure. People thought the smoke had caused the deaths, perhaps exacerbated by the acid, but today, we know that the acid was the main cause of death. In the Great Smog of 1952, the human bodys defences were overwhelmed by the severe conditions present in London.

The Bhopal disaster that killed several thousand people and injured another two lakhs within a matter of a few hours constitutes a watershed in the history of the chemical industry. The first of the autopsies revealed that the human blood had turned purple red; the lungs had become ash colour and filled with their own secretions. The tracheas were so dry that the mucous flaked off on touch.

In cases of acute exposure, victims had suffered extensive damage to their lungs. Those who did not succumb to their injuries, fell victims to secondary infections of the lungs and respiratory tracts. The psychological trauma caused by the accident is just beginning to be acknowledged and goes far beyond those physically affected by the gas.

A series of studies made five years later showed that many of the survivors were still suffering from one or several of the following ailments: partial or complete blindness, gastrointestinal disorders, impaired immune systems, post traumatic stress disorders, and menstrual problems in women. A rise in spontaneous abortions, stillbirths, and offspring with genetic defects was also noted.

Not much is yet known about the environmental impacts of the gas leak from the Bhopal plant. The Indian Council of Agricultural Research (ICAR) had issued a preliminary report on damage to crops; vegetables, animals and fish from the accident, but the investigation reported there were mostly in their early stages with few conclusive findings.

The ICAR Report did indicate that the impact of toxic substances emerging from the plant were highly lethal on exposed animals. Many were reported to have died within three minutes of such exposure. Large numbers of cattle (estimates range as high as 4000), as well as dogs and cats and birds were killed.

Plant life was also severely damaged by exposure to the gas. Vegetable crops such as spinach, cauliflower and tomatoes grown by small farmers on the outskirts of the city were destroyed. There was also widespread defoliation of trees, especially in low-lying areas.

In case of Meuse Valley, the chief symptoms of the affected were dyspnea, either in the form of asthma-like attacks with labored expiration, or continuous polypnea. In the fatal cases, an acute circulatory insufficiency occurred, with rapid and poor pulse, face pale - more rarely cyanotic - and an extension of the cardiac dullness. Examination of the lungs showed signs of bronchitis. Other symptoms included changes in the tone of the voice, increasing to hoarseness, nausea, occasional vomiting, and lachrymation. Animals were also affected; cattle showed the symptoms of increased and superficial respiration, uneasiness, acute emphysema, cyanosis of the mucous membranes, sometimes death.

In Poza Rica episode, the most frequent symptom was the loss of the sense of smell. More than half of the patients lost consciousness, many suffered signs and symptoms of respiratory tract and eye irritation and some developed pulmonary oedema. Four of the 320 victims exhibited neurological sequelae; 2 experienced neuritis of the acoustic nerve; 1 developed dysarthria; the fourth patient suffered aggravation of pre-existing epilepsy.

The aftermath of these disasters led to serious public concern leading to various pollution control acts and awareness about environmental health and safety. Relationships between air pollution emissions and exposure are now better understood through studies of atmospheric dispersion, chemical reactions and deposition of pollutants. Pollution management including assessment, control, and surveillance has also become more sophisticated.


In the name of economic development, Third World countries are becoming dumping grounds for hazardous technologies from the industrially advanced countries. Hazardous chemicals that are banned in developed countries are being pushed into the Third World countries. The strong environmental awareness and environmental movements in the industrially advanced countries have enforced strict legislative safeguards that have made the operation of hazardous technology economically unviable.

The need to take environmental considerations into account to ensure successful economic development is being recognized throughout the world. The introduction and use of environmentally friendly practices and technologies in many industrialized countries have decoupled pollution from economic growth. Developing countries are also taking steps to protect their environment, although few, if any, can afford the large-scale investments in cleaner air and water.

In spite of the advances made in technologies in the field of pollution control, a lot remains to be done. Environmental philosophers have time and again warned us about the dangers of pesticides, atomic energy plants and similar phenomena of modern industrialization. However, man in his pursuit of economic development often tends to ignore the environmental risks involved. When a nightmarish incident like the Bhopal tragedy involving loss of numerous lives occurs, our thoughts then return to the basic question :IS THIS THE PRICE WE ARE PAYING IN THE NAME OF PROGRESS?