Pollution & Environmental Health: The Dose, the Plume and the Count

On 5 December 1952 a cold, still fog settled over London and stayed for four days. Coal smoke from millions of chimneys had nowhere to go. Hospitals filled with people who could not breathe, and undertakers ran out of coffins. Contemporary counts put the deaths at about 4,000, and later analyses of the death records suggest it was several times that, up to roughly 12,000. The Clean Air Act of 1956 followed. The smog came from a fuel, a chimney and the weather, and what it did to people could be counted. All of environmental health is that chain: a source, a path, a dose and a count.

The chain is still running. The Lancet Commission on Pollution and Health estimated that pollution caused about 9 million deaths in 2015, and its 2022 update, using 2019 data, found about 9 million again, around one death in six worldwide. It put air pollution at about 6.7 million of those, water pollution at about 1.4 million and lead at about 0.9 million, with most of the burden in low- and middle-income countries. These are modelled estimates with wide uncertainty, built the way Epidemiology & Evidence-Based Medicine (35/58) teaches, and this piece shows you how.

This piece explains how environmental health works. It is not medical advice. For a health question, see a clinician; in an emergency, call your local emergency number.

The dose makes the poison, and the curve's shape matters

"All things are poison, and nothing is without poison; only the dose makes a thing not a poison", wrote Paracelsus in the 1530s. Water, salt and vitamin A can all harm in excess. Toxicology turns the saying into a graph, the dose–response curve: dose along the bottom, the chance or size of harm up the side. What a graduate remembers is that the whole argument is at the left-hand end, where doses are low and effects are too rare to see in any one study.

There are three common shapes. A threshold model says nothing happens until a dose is passed (the body repairs or clears small amounts). A linear no-threshold (LNT) model says every bit adds risk in proportion. A saturating curve rises fastest at low doses and flattens later. Regulators typically assume no threshold for substances that damage DNA and for the dose levels they cannot measure, as a precaution. Critics, including supporters of hormesis (a claim that low doses of some agents are beneficial), say this overstates low-dose risk; supporters of precaution reply that the data at the low end are too sparse to rule it out. For fine particles the WHO states that no safe level has been identified, and the curves used in global burden studies are typically concave: steepest at low levels.

The multiplication at the bottom is the second lesson. Epidemiologist Geoffrey Rose pointed out in 1981 that most cases of a disease come from the many people at moderate risk, not the few at high risk, simply because there are more of them. Try it below.

With the no-threshold curve, a dose of 10 gives 20 extra cases per 100,000, so 10 million people give 2,000. The 1% at dose 80 (100,000 people) produce 160 cases; the other 99% at dose 10 produce 1,980. The few high-dose people are the visible emergency, but the many at lower doses are most of the count. With a threshold of 20, dose 10 produces none, which is why the shape of the left-hand end is worth arguing about.

Air: PM2.5, the guideline and who breathes what

PM2.5 is particulate matter 2.5 micrometres across or smaller: soot, smoke, sulphate and nitrate droplets, and dust, from burning fuel in engines, power stations, cooking fires and wildfires, and formed in the air from gases such as sulphur dioxide. A hair is about 70 micrometres wide, so particles this small slip past the nose and throat, reach deep into the lungs and carry some of their material into the blood. They are linked to heart disease, stroke, lung disease and cancer, which is why fine particles, not the more visible coarse dust, dominate the death counts.

The World Health Organization's Global Air Quality Guidelines of September 2021 cut the annual PM2.5 guideline from 10 to 5 µg/m³ and the 24-hour value from 25 to 15 µg/m³, and cut the annual nitrogen dioxide guideline from 40 to 10 µg/m³. Guidelines are not laws. Countries set legal limits that balance health against cost and feasibility, and the legal limits differ widely: the annual PM2.5 figures are 9 µg/m³ in the United States (set in 2024), 10 µg/m³ from 2030 in the European Union's revised air quality directive, 35 µg/m³ in China's standard for most areas (a stricter 15 for the cleanest zones), and 40 µg/m³ in India's national standard. Each can be justified, and none is the WHO number. The State of Global Air programme's 2025 report linked air pollution to roughly 7.9 million deaths in 2023, about 4.9 million from outdoor air and 2.8 million from household air pollution, mostly from cooking with solid fuels. These are estimates from a different group with a different method from the Lancet Commission's, and the totals differ for that reason.

How do we know the particles themselves are the cause, not the smoking, income or climate of the places that have them? That is the confounding problem of Epidemiology & Evidence-Based Medicine (35/58). The Harvard Six Cities study (Dockery and colleagues, 1993) followed adults in six US cities for about 15 years and found that, after adjusting for smoking and other factors, mortality in the most polluted city was about a quarter higher than in the cleanest. Natural experiments reinforce it: studies of a steel-mill closure in Utah's valleys in 1986 and 1987 found fewer admissions for children's respiratory disease, and cities that cleaned their air saw life expectancy rise.

At 100 µg/m³ the air is 20 times the WHO guideline, and even India's standard, the loosest here, is exceeded 2.5 times. At 8 µg/m³ the air meets every limit in the table but the WHO's. Because the curve has no known safe floor, the guideline is not a line between safe and unsafe. It is a point on a slope that the WHO judged achievable and protective.

How far a plume goes

A concentration in the air depends on how much is released, how high, and how fast it mixes. Turbulence spreads a plume sideways and upward in a bell-shaped (Gaussian) pattern, and the widths grow with distance at rates that depend on the weather. Meteorologists sort the weather into stability classes: unstable (sunny, with rising thermals), neutral (overcast, windy) and stable (clear night, still, often with an inversion). At ground level, directly under the plume, the concentration is:

Read it as two ideas. Dilution: more wind, or a wider plume, means a lower concentration. Height: the exponential term keeps the plume off the ground until it has spread to about the height H. Tall stacks were built to exploit this, and they worked locally, but taller stacks without cleaner fuel moved sulphur from nearby towns to far-away forests and lakes, one source of the acid rain that polluted Scandinavia and eastern North America from the 1970s. And on a stable night a lofted plume can stay aloft for kilometres and then come down far away.

With a 50 m effective height and a 3 m/s wind, the overcast case peaks at about 1,600 µg/m³ about 800 m away. In sunny, unstable air the same chimney peaks higher (about 2,400) but closer (about 300 m), because turbulence brings the plume down quickly. On a clear night the peak is lower (about 600) but 4 km away, and at 1 km the ground sees almost nothing: the people nearest to the stack are spared and those farther off are not. Raise the wind to 8 m/s and the peak falls to about 600 for the neutral case. Dilution is the cheapest pollution control there is, and it fails when the air will not move.

Persistent chemicals and the food-chain ladder

Some chemicals vanish in days; others stay for decades. Two properties decide which become problems: persistence (the half-life in soil, water or bodies) and fat-solubility, measured by log K_ow, the octanol–water partition coefficient. A fat-loving chemical leaves water for the fat of living things. A rule of thumb from the 1970s relates the two: log BCF ≈ 0.85 log K_ow − 0.70, where the bioconcentration factor (BCF) is how many times more concentrated the chemical is in a small aquatic organism than in the water. Then, because fat-bound chemicals are not excreted, each meal passes them to the next consumer along with the food, so the concentration rises up the chain, biomagnification, by a factor per step (a biomagnification factor, BMF).

The classic case is DDT. Rachel Carson's Silent Spring (1962) argued that it thinned the eggshells of birds of prey and killed songbirds, and the United States banned most uses in 1972. DDT is also a reminder that the story has a second side: it saved huge numbers from malaria, and the Stockholm Convention (adopted in 2001, in force from 2004) still allows it for disease-vector control in some countries. Mercury is the other textbook case, with the Minamata disease of the 1950s in Japan and the global Minamata Convention (adopted 2013, in force from 2017). The modern concern is PFAS, the "forever chemicals": thousands of compounds that resist breakdown. In 2024 the US EPA set drinking-water limits of 4 nanograms per litre for two of them, PFOA and PFOS, though the schedule has been under review since.

At log K_ow 6, BCF is about 25,000, and a factor of 3 per step puts the top bird at about 680,000 times the water concentration. A chemical with log K_ow of 3 and the same factors is magnified only about 2,000 times. After 20 years with a 5-year half-life the water is at 6% of the start and the whole ladder with it, which is why banned persistent chemicals take generations to clear and why a short half-life is the best property a new chemical can have.

Lead: one metal, a century of evidence

Lead is the best-studied pollutant in history, and its lesson is about speed. Thomas Midgley Jr., at General Motors, found tetraethyl lead to be an effective anti-knock additive for petrol on 9 December 1921, and leaded petrol spread across the world. The geochemist Clair Patterson, measuring the age of the Earth in the early 1950s, found his samples contaminated by environmental lead, traced it in ice cores to the arrival of leaded petrol, and became an early and effective campaigner against it. The US phased down lead in petrol from 1976 and banned it for on-road vehicles from 1 January 1996, and Algeria, the last country to sell it, stopped in July 2021; the UN Environment Programme declared the end of leaded petrol on 30 August 2021.

The harm is well documented, and it falls hardest on children. Lead damages the developing brain, and the World Health Organization states that no level of exposure has been identified as safe. In the US the CDC's blood lead reference value, the level at which children are flagged for follow-up, was lowered to 3.5 micrograms per decilitre in 2021, from 5. A 2020 UNICEF and Pure Earth report estimated that around one child in three, up to 800 million worldwide, has a blood lead level at or above 5 micrograms per decilitre, with the main sources including battery recycling, old paint, spices, cookware and pipes. In Flint, Michigan, the city switched its water source in April 2014 without adding corrosion control, and lead leached from old pipes: a failure of treatment, covered further in Environmental Engineering (34/68).

Two points matter for the rest of the course. First, lead was an enormous natural experiment: blood lead fell with the phase-out in the countries that did it, which is strong evidence that policy works. Second, the evidence kept hardening for decades at lower and lower doses, as measurement improved, and each time the "safe" level moved down. That is what a no-threshold-in-practice pollutant looks like.

Plastics: counting what is hard to count

The scale is counted best in a 2017 paper by Roland Geyer, Jenna Jambeck and Kara Lavender Law in Science Advances: about 8.3 billion tonnes of plastic produced from 1950 to 2015, of which about 6.3 billion tonnes became waste. Of that waste, roughly 9% was recycled, 12% incinerated, and 79% accumulated in landfills or in the natural environment. In an earlier paper, Jambeck and colleagues estimated that 4.8 to 12.7 million tonnes entered the oceans from land in 2010; a 2021 study of river inputs (Meijer and colleagues) estimated 0.8 to 2.7 million tonnes a year, concentrated in a thousand or so rivers. The estimates differ because plastic leaks in many ways and is measured in few.

Fate of plastic waste to 2015	Share	Billion tonnes (of 6.3)
Recycled	9%	0.57
Incinerated	12%	0.76
Landfill or the environment	79%	4.98

What this does to health is the open question. Microplastics (under 5 mm) and nanoplastics have been found in human blood, placentas and arteries; a 2024 study in the New England Journal of Medicine found plastic particles in the fatty deposits of the carotid artery in many patients, and the patients who had them had more heart attacks, strokes and deaths over about three years. That is an association, not proof of cause, and methods for measuring tiny plastic particles in tissue are being challenged. The additives in plastics (some phthalates, bisphenols and flame retardants) have clearer evidence of harm than the plastic itself. Governments are negotiating a treaty. The talks in Geneva in August 2025 ended without agreement on a text, so check the UN Environment Programme for where the talks stand now.

What a graduate just sees

• Every pollution story is a chain: source, path, dose, harm. Break any link and the harm stops.
• The shape at the low-dose end of the curve is the real argument, and it is hard to measure.
• A small risk times a large population is a big number; most cases come from the many at moderate risk.
• Guidelines (WHO) and legal limits differ; a limit is a judgement about cost, not a line between safe and unsafe.
• Dilution works until the air stops moving, and a taller stack moves a problem.
• Persistent plus fat-soluble means it builds up along food chains; half-life decides the clean-up time.
• For lead, safe levels were revised down again and again; the best evidence for regulation is the fall after bans.
• Death counts for pollution are modelled estimates from several groups, with different methods and wide ranges.

Where it shows up next

• Environmental Economics & Policy (36/19): pollution is the textbook externality; limits, taxes and trading schemes put a price on the dose.
• Public & Global Health (35/61): burden-of-disease counting and the household-air-pollution programmes behind the cooking-fire numbers.
• Soils, Agriculture & Food Systems (36/18): fertiliser and manure are the largest source of water pollution in many places.
• In the world: air-quality alerts, drinking-water standards, chemical regulation (REACH in the EU, TSCA in the US) and the engineers who run treatment works.

If you want to go deeper

• 📘 Textbook. Gilbert Masters and Wendell Ela, Introduction to Environmental Engineering and Science (Pearson, 3rd edition 2008). A standard undergraduate text for dispersion, risk and water.
• 📘 Textbook. Howard Frumkin (ed.), Environmental Health: From Global to Local (Jossey-Bass/Wiley, 3rd edition 2016). The public-health view of exposures from air to climate.
• 📘 Gentler. Rachel Carson, Silent Spring (1962), the book that began modern environmental regulation, still readable as science writing.
• 🌐 Free course. MIT OpenCourseWare 20.104J / BE.104J, Chemicals in the Environment: Toxicology and Public Health (Spring 2005), covering toxicology, epidemiology and risk assessment.