Air Pollution's Discovery: A Historical Perspective

Air pollution has been a problem for thousands of years, but the discovery of its dangers and its causes is a more recent phenomenon. From wood fires in ancient homes to the Industrial Revolution, humans have been polluting the air for millennia. The recognition of air pollution as a threat to human health and the environment began with early societies, and the issue was considered by Roman courts 2,000 years ago. However, it is only in the last 150 years that high-quality measurements of air pollutants have been available, and the last 40 years that numerical modelling has been possible. The development of satellite remote sensing in the 21st century has also provided valuable global data on the distribution of primary and secondary pollutants.

Air pollution recognised as a threat to human health since 400 BC

Air pollution has been a problem for thousands of years, but it was first recognised as a threat to human health around 400 BC, during the time of Hippocrates.

In the millennia that followed, successive written accounts of air pollution emerged from different countries, with the first measurements of air quality taking place in the 18th century. These measurements showed the growing scale of poor air quality in urban centres and close to industry, as well as the chemical characteristics of the gases and particulate matter.

The ancient Romans, for instance, experienced air pollution in the form of smoke, which they referred to as "gravioris caeli" ("heavy heaven") or "infamis aer" ("infamous air"). Roman courts considered civil claims over smoke pollution 2,000 years ago, and the empire even tried an early version of the Clean Air Act. In 535, Emperor Justinian proclaimed the importance of clean air as a birthright, writing, "By the law of nature, these things are common to mankind—the air, running water, the sea".

The Industrial Revolution, which began around 1820, accelerated the magnitude of emissions of primary pollutants and the geographical spread of contributing countries. The shift to fossil fuels and the use of steam engines in centralized factories eliminated constraints on urban expansion, leading to the emergence of large industrial cities that pumped vast amounts of pollution into the atmosphere. By 1900, there were 43 cities worldwide with a population of more than 500,000 people, up from just six in 1800. Residents of these emerging industrial hubs experienced acrid smoke that stung their eyes and hindered their breathing.

The Great Smog of London in 1952 was one of the most severe events related to air pollution in the city's history. For four days, the UK's capital was covered in a thick layer of lethal smog, primarily caused by coal combustion. This event led to the adoption of the Clean Air Act four years later, which aimed to tackle air pollution from industrial activities.

Industrial Revolution accelerated emissions and geographical spread

Air pollution has been a concern since ancient times, with written accounts of it dating back to the time of Hippocrates, around 400 BC. However, the Industrial Revolution, which began in Britain in the late 18th century, played a significant role in accelerating emissions and their geographical spread.

The shift to fossil fuels, particularly coal, during the Industrial Revolution, led to a sharp increase in carbon emissions. Coal was used to power new factories, ships, and trains, and it was also used for smelting iron and providing heat for homes. This heavy reliance on coal caused a substantial rise in CO2 levels, with deforestation also contributing to the release of greenhouse gases. The burning of coal also increased emissions of pollutants such as SO2, NO2, NH3, and smoke. As a result, cities during the Industrial Revolution, such as Birmingham, Leeds, Manchester, Chicago, and Pittsburgh, experienced acrid smoke and thick fogs that stung the eyes and hindered breathing.

The Industrial Revolution also brought about urbanization and industrialization, with cities expanding rapidly to accommodate the growing workforce needed for factories. These urban centers became heavily industrialized and increasingly polluted, with poor air quality becoming a defining issue. The rapid industrial growth outpaced the ability to manage waste and emissions, resulting in long-term environmental and public health impacts.

The Second Industrial Revolution, from the late 19th to early 20th centuries, introduced further advancements such as electricity, steel production, and the internal combustion engine. This phase drove mass production and the growth of heavy industries across Europe and North America, leading to exponential increases in energy consumption and resource extraction. The cumulative environmental strain of each phase of industrialization caused significant increases in air pollution, deforestation, and greenhouse gas emissions.

The effects of the Industrial Revolution on air pollution were not limited to the immediate period. It set in motion a pattern of unchecked resource exploitation and high emissions that continue to impact the world today. The large-scale carbon emissions from the Industrial Revolution contribute to ongoing global warming, and the environmental consequences, such as deforestation and loss of biodiversity, remain major global challenges.

Roman courts considered civil claims over smoke pollution 2,000 years ago

Air pollution has been a concern for humans for millennia. As early as 400 BC, Hippocrates recognised air pollution as a threat to human health, and written accounts of air pollution from different countries emerged over the following two thousand years.

Indeed, Roman courts considered civil claims over smoke pollution two thousand years ago. The jurist Aristo, for instance, ruled that a cheese shop could not discharge smoke into the buildings above it, as the fumes were causing a nuisance to residents. The Roman Empire even attempted an early version of the Clean Air Act. In 535, Emperor Justinian proclaimed the importance of clean air as a birthright, stating:

> "By the law of nature, these things are common to mankind—the air, running water, the sea."

After the decline of the Roman Empire, emissions decreased temporarily. However, with the advent of smelting to create lead and copper, the air quality in medieval times suffered. Analyses of ice cores from the Arctic revealed that extraction and smelting activities in regions such as the Iberian Peninsula, England, and Greece led to a tenfold increase in lead pollution.

The Industrial Revolution further exacerbated air pollution, with large-scale factories and the shift to fossil fuels contributing to the problem. By 1900, there were 43 cities worldwide with a population of over 500,000, and residents of these burgeoning industrial centres experienced the negative effects of smoke and pollution on their health and well-being.

Advances in detection and understanding of air pollution in the 21st century

Air pollution has been recognised as a threat to human health for thousands of years, from the time of the ancient Romans to the Industrial Revolution. However, it is only in the last 150 years that we have had the technology to take high-quality measurements of air pollutants. The 21st century has seen significant advances in the detection and understanding of air pollution, building on the work of scientists in the 20th century who discovered the links between air pollution and phenomena such as acid rain, eutrophication, and ground-level ozone.

At the beginning of the 21st century, new measurements and satellite remote sensing technologies provided global data on the distribution of primary and secondary pollutants. This, along with new epidemiological evidence, brought the effects of air pollution on human health to the top of political agendas worldwide. The development of portable, lower-cost air pollution sensors that report data in near-real time has been a game-changer in air pollution monitoring. These sensors provide individuals and communities with the tools they need to understand their environmental exposures and develop strategies to reduce pollution exposure and improve health outcomes.

Another important development in the 21st century has been the use of unmanned aerial vehicles (UAVs) for air pollution measurements. Satellite observations are also a growing field, with increasingly small and cost-effective platforms, such as nanosatellites, providing new opportunities for global measurements of air pollution. These technologies have helped to fill in the gaps in our understanding of air pollution, which is critical for developing effective policies and interventions to improve air quality.

In addition to technological advances, our understanding of the health impacts of air pollution has also improved. For example, studies in the 1990s showed a link between PM10 and "chronic" mortality, drawing particular attention to the health effects of fine particles. More recently, the 2012 National Academy of Sciences (NAS) report, "Exposure Science in the 21st Century: A Vision and a Strategy", identified ubiquitous sensors as a technology that could substantially enhance exposure science and provide more accurate and comprehensive personal exposure data. This improved understanding of personal exposure to air pollution is critical for developing effective air quality management policies.

Despite these advances, air pollution remains a significant problem, with 91% of the world's population living in places where air pollution levels exceed the World Health Organization's guideline limits. As we continue to grapple with the challenges of air pollution, further research and innovation will be needed to improve air quality and protect human health and the environment.

Air pollution a consequence of industrial growth and urban expansion

Air pollution has been a concern since the time of Hippocrates, around 400 BC, and written accounts of it occur over the following two millennia. However, the Industrial Revolution accelerated the magnitude of emissions and the geographical spread of contributing countries, with highly polluted cities becoming a defining issue. The shift to fossil fuels and centralized factories on a large scale removed constraints on urban expansion, and the burning of coal, gasoline, and oil in vehicles, homes, industries, and power plants has been a major source of harmful nitrogen oxide (NOx) emissions.

The growth of cities has been a significant factor in the increase in air pollution. In 1800, there were just six cities worldwide with more than 500,000 people. By 1900, there were 43. Residents of emerging industrial powerhouses like Birmingham, Leeds, Manchester, Chicago, Pittsburgh, and St. Louis experienced acrid smoke that stung their eyes and hindered their breathing. Thick fogs, especially in colder weather, blanketed these cities. The famous London Smog of 1952 led to the deaths of an estimated 3,000–4,000 people and resulted in the Clean Air Act of 1956.

The problem of air pollution has continued to worsen with further industrialization and urban expansion. Chongqing, China, for example, became a prominent megacity, experiencing urban expansion, extensive construction, improvements in transport infrastructure, a rise in private car use, and population increases. The urban population rose from 17.9% of the total population in 1978 to 51.3% in 2011. During this period, Chongqing also developed into a major manufacturing center for automobiles, steel, aluminum, and chemicals, with intense economic growth fueled by coal combustion. The impacts of urban expansion on air quality have been observed to be of the same order as those induced by emissions growth.

The effects of air pollution on human health have been well-known for centuries, but new measurements, satellite remote sensing, and epidemiological evidence have brought the issue to the forefront of political agendas worldwide. Despite a 90% decline in SO2 emissions in Europe and North America from their peak in the 1970s, new pollutants have been discovered that are responsible for millions of premature deaths and health issues worldwide. According to the World Health Organization (WHO), 91% of the world's population currently lives in places where air pollution levels exceed the organization's guideline limits.

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