Air pollution is taking lung tissue, not just irritating it — and the pollutant still doing it is ozone

In June we published a piece here saying that recent histological analysis of urban residents shows fine particulate pollution permanently remodelling lung tissue. We named no study, no cohort and no number. There is good human evidence for something close to that claim, and when you read it, it points somewhere we did not: at ozone.

What the best human study measured

The Multi-Ethnic Study of Atherosclerosis followed 7,071 adults aged 45 to 84 across six American metropolitan regions from 2000 through November 2018. Participants were scanned up to five times each. The measure was percent emphysema — the share of lung on a CT scan below −950 Hounsfield units, which is the radiological signature of alveolar walls that are simply no longer there. The results were published in JAMA in August 2019.

Emphysema is not inflammation and it is not scarring. It is absence: the tiny air sacs where the lung hands oxygen to the blood are destroyed, and the lung does not build them back. Whatever else is arguable about air pollution and health, this endpoint is not reversible.

The numbers

Median percent emphysema was 3% at the start and rose by an average of 0.58 percentage points per decade — the background rate of getting older. Against that baseline, each 2 micrograms per cubic metre of PM2.5 at study entry was associated with an additional 0.11 percentage points per decade, with a confidence interval of 0.03 to 0.19. Each 3 parts per billion of ozone was associated with 0.13, interval 0.03 to 0.24. Oxides of nitrogen and black carbon showed smaller but similar associations.

So the effect is real and it is modest per unit: roughly a fifth added to the ordinary rate of decline for each 2 micrograms of particulate. It is not a lung destroyed in a year. It is a lung that ages faster, measurably, for decades.

The finding we would have missed

Here is the part that changes the story. PM2.5 and nitrogen oxides fell substantially across the United States during the follow-up period. When the authors looked at exposure DURING follow-up rather than at baseline, ozone and NOx were still significantly associated with emphysema progression — and PM2.5 was not.

Read plainly: the particulate problem the original piece was written about is the one America has been fixing. The pollutant still doing this damage at contemporary concentrations is ozone, which has not fallen, because it is not emitted directly. It is manufactured in the air on hot, still, sunlit days out of traffic and industrial precursors.

Which is the Sacramento story

The Sacramento Valley is one of the country's harder ozone problems and for structural reasons: a bowl with mountains on three sides, long hot cloudless summers, and an inversion that holds the day's chemistry in place. The ingredients arrive in the morning commute and cook through the afternoon. On the worst days the peak arrives downwind, in the foothills, hours after the traffic that made it.

If the JAMA result holds, that is not a nuisance about visibility on a hot afternoon. It is the pollutant most associated with lungs that are quietly losing tissue, sitting over a region that has some of the highest summer concentrations in the country.

What this does not show

This is an observational cohort, not an experiment: it establishes association across a large, long, well-measured population, and it cannot assign cause in any one person. Exposure was modelled from residence, so anyone who spent their days somewhere other than home is measured imperfectly. And a CT-based percentage is a population instrument — it says nothing useful about a single scan.

What it does establish is that the endpoint is structural rather than symptomatic, that it accumulates over decades, and that the pollutant still driving it at today's concentrations is the one this valley is worst at.