The coroner pronounced in 2020 that the toxic fumes she had breathed because of the traffic on the roadway were partly to blame. It was the first known instance of the law recognising air pollution as a cause of death.

Yet public health experts believe that Kissi-Debrah was a victim of a far more widespread, global emergency. The World Health Organization (WHO) estimates that air pollution is responsible for more than seven million premature deaths every year around the world, contributing to pulmonary and heart diseases, lung cancer and respiratory infections. Almost all of the world’s population – 99 percent – breathes air that is dirtier than levels recommended by the WHO.

In August, research published in The Lancet journal by a team of Chinese scientists showed that air pollution increases antibiotic resistance, which – their calculations suggest – in turn led to 480,000 premature deaths and 18 million years lost globally in 2018.

The same month, Harvard scientists showed an association between pollutants in emissions from carbon combustion and an increased risk of some cancers.

So, just how dangerous is the air we breathe?

The short answer: Very. In fact, air pollution, by some estimates, is a leading cause of death internationally. Carbon emissions are down, but wildfires are up, and the global hunger for energy continues to mount, posing new threats that do not affect everyone equally. Like with many other public health challenges, where people live and how much they earn determines the risks they face from toxic air.

Gas and particles

Airborne pollutants principally fall into two categories, said Sophie Gumy, a technical lead for the WHO’s Department of Environment, Climate Change and Health: gases and particulate matter that are either produced directly from carbon combustion or through secondary mechanisms.

Nitrous oxides – a group of gases commonly produced by vehicles, fossil fuel-based power production, industrial refineries and chemical plants – are an example.

As a primary pollutant, nitrogen dioxide has been shown to exacerbate asthma and respiratory conditions. However, nitrogen dioxide, along with other nitrogen oxides, can also interact with the sun’s ultraviolet light and a category of gases known as volatile organic compounds to produce secondary pollutants like ground-level ozone, which can pose health problems such as inflammation and damage to the airways. This evolution of pollutants also complicates the problem of understanding where to stem the tide.

These same nitrogen oxides can also contribute to the formation of airborne particulates. And though both the ozone and the particle originated from the same sources, such as tailpipe nitrogen dioxide – their effects and magnitude are markedly different.

“The particulate pollutants are the ones that are particularly important for public health,” said Scott Budinger, chief of pulmonary and critical care at Northwestern University Feinberg School of Medicine. In fact, he notes, research has found that following just days after upswings in particulate exposure, there is often an increase in deaths from a wide range of health complications.

“What epidemiologists have noted,” said Budinger, “is that if you look at daily particle exposure in any large, urban city, you’ll see about a two- to three-day lag, then an increase in all-cause mortality.”

The biggest burden is on the heart.

“People think ‘air pollutant’,” said Gumy, “and they think ‘respiratory problems’.” However, she said, particulates, taken up by the lungs and transported through the bloodstream, cause inflammation in cells. This strains the heart, contributing to cardiovascular issues, such as heart attacks and strokes.

The worst offenders are particles smaller than 2.5 microns, approximately 20-28 times smaller than the diameter of a human hair. At that size, the particulates can even get into the placenta, and thus, the brains of foetuses, said Maria Neira, director of the WHO’s Department of Environment, Climate Change, and Health.

Several studies have even found high exposure to these fine particles, known as PM2.5, to be a significant risk factor for premature birth. Long-term PM2.5 exposure has also been associated with an increased risk of developing neurodegenerative diseases such as dementia, Parkinson’s and Alzheimer’s.

And often, the worst sufferers are already disadvantaged communities.

Deadly exposure

According to the Institute for Health Metrics and Evaluation at the University of Washington in Seattle, air pollution is the fourth-leading cause of mortality among all metabolic and behavioural risk factors, after high systolic blood pressure, tobacco use and dietary risks.

But that toll is not distributed equally.

According to WHO, Indoor air pollution is associated with 3.2 million deaths a year. Most of that is in regions such as Sub-Saharan Africa, the Indian subcontinent, several Southeast Asian countries and Russia, where many households still use dirtier-burning fuels such as kerosene, wood or coal for primary heating or cooking.

The associated increase in stroke, ischaemic heart disease, chronic obstructive pulmonary disease (COPD), and lung cancer tend to hit women and children, who traditionally tend to do more household chores, the hardest.

Outdoor, or ambient, air pollution is associated with 4.2 million premature deaths worldwide, according to the WHO. Last year, the World Bank estimated the global toll of the health damages associated with ambient air pollution to be $8.1 trillion – roughly 6.1 percent of the global gross domestic product.

However, again, much of that burden falls on low and middle-income populations. One analysis estimated that, if air pollution could have been kept to theoretical minimum levels across the Middle East and North Africa in 2019, the average life expectancy would have been higher by anywhere between one and six years.

Air pollution’s effects are felt most by susceptible and vulnerable populations, said Gumy. A susceptible population is a group likely to be hardest hit by the effects of air pollution: Those with preexisting conditions, or genetic predispositions to respiratory or cardiovascular issues, for instance. The vulnerable, on the other hand, are those most likely to be exposed in the first place. Often, those who live in poorer areas fit both criteria. They may live or work jobs that expose them more, while also having less access to health care and less time to stay active and fit.

Picture a police officer in India’s capital New Delhi, said Gumy. Every winter, farmers from neighbouring states burn their fields, triggering a months-long season of toxic air pollution. An officer patrolling their beat outside has no choice but to be exposed throughout the day.

Even in high-income countries, groups in lower socioeconomic brackets tend to be disproportionately affected. In the United States, Budinger pointed out, such communities often live closer to roadways. This saddles many minority groups, especially Black Americans, with a higher incidence of premature death from associated medical conditions.

Socioeconomics aside, geography influences who gets most affected by air pollution. Generally, said Budinger, the cities most affected are ones in valleys, where air can often get trapped.

Likewise, with the progression of climate change, areas most at risk for forest fires face significant challenges to air quality.

According to US Environmental Protection Agency data, the country has seen a 42 percent decline in PM2.5 nationally over the last two decades. However, in that time, the incidence of wildfires has tripled. The average fire size has quadrupled. And so despite the national decline, Pacific Northwest communities face rising particulate levels, with incidents of heavy exposure. One Stanford University analysis found that from 2007 to 2019,  wildfires accounted for 20 percent of PM2.5 emissions in the US, racked up $92bn in annual welfare tolls, cost between $8bn and $31bn in premature mortality-related expenditures, and reduced labour earnings by nearly 2 percent.

“Every day,” said the WHO’s Neira, “we are getting more evidence about how air pollution is affecting [us].”

A recent analysis of 12 research studies conducted across 116 countries found that every 10 percent increase in PM2.5 was associated with a roughly 1 percent increase in antibiotic resistance, equating to 43,654 deaths, with the highest levels of resistance seen in North Africa and West Asia.

“We need to make sure that everyone understands that air pollution is representing probably one of the biggest public health challenges that we are confronting today,” said Neira.

To be sure, the health effects of air pollution are still a relatively new area of research – with many questions unanswered. What is clear, though, is that the road ahead is bumpy.

Good news and bad

There have been victories in the battle against air pollution.

The EPA in the US estimates that the Clean Air Act has saved hundreds of thousands of lives across the country in the 50 years since it was passed.

Thanks in large part to the 1979 Convention on Long-Range Transboundary Air Pollution (LRTAP), which created a regional framework to address air pollution across Europe, North America, the former Soviet Union and its Warsaw Pact allies, a number of European countries have seen a dramatic decline in air pollution, especially for sulphurous emissions.

“For a very long time,  air pollution was considered an urban problem,” said the WHO’s Gumy. “But the science really allows us to analyse where the pollution comes from.”

For example, global emissions of nitrous oxides, sulphur dioxide, carbon monoxide, black carbon and atmospheric organic carbon have all been on the decline since 2012 or earlier, driven by reductions from China, North America and Europe.

Progress is not restricted to wealthier countries either. Emissions reductions in the transport sector in India and Africa, as well as a drop in pollutants from India’s energy sector, account for 60 percent of the global fall in emissions of nitrous oxide and 14 percent of sulphur dioxide.

However, it isn’t all good news. Year after year, global energy consumption continues to increase. Ammonia emissions steadily rise due to farming. Many volatile organic compounds are becoming increasingly common due to solvent use, energy production and traffic.

Moreover, Gumy pointed out, as many low or middle-income countries go through different development patterns than wealthier countries, they will likely see a mixture of both old and new sources of air pollution – with soot released from burned residential waste rising alongside nitrous oxide from growing car ownership.

The reality is that the road ahead is not one road – it is a multitude of different paths for different regions that crisscross in unpredictable ways.

A region could be a leader in improving its air quality through new standards for industrial emissions, while others could be lagging. A country doing well could also see its efforts poleaxed by soot if its neighbouring nation decides to clear forests. That is true for different neighbourhoods, cities and states within a country too.

And while the patchwork of monitoring services available in different regions makes it possible for people to monitor their air quality with some degree of accuracy, that is not good enough for the WHO, said Neira – especially in the face of the new health risks of air pollution emerging every day.

People need to understand what the pollution data means, “but not only by putting the responsibility on them to say, ‘Okay, I have an app that tells me the street is polluted’”, she said. “This is not the way to fight pollution.”

“You need to have the right to breathe air that is not killing you on a daily basis.”