A nurse in Ohio opens a chart and can't account for the week. More breathless. Poor sleep. Didn't get out at all. Nothing changed clinically, same regimen, same weight, same everything. It reads like deterioration with no cause.

The cause was four hundred miles upwind, in northern Ontario, and it will never appear anywhere in that patient's record.

The summer both continents just had

In the week of 17 July, smoke drifting south from Canadian wildfires put more than 100 million Americans under air-quality alerts, from Milwaukee to Washington DC, hazing the New York skyline and sinking to ground level across the Great Lakes and Northeast. By mid-July, roughly 8.6 million acres had burned in Canada season-to-date, already above the 1983-onward average of about 6.5 million, with more than 800 fires burning at once.

Europe ran the same story on its own fuel. Copernicus reported France's year-to-date fire emissions as comparable to or higher than the extreme seasons of 2022 and 2023, on course to be one of the most severe fire years in 24 years, with Spain well above average and the EU civil protection mechanism activated for both. The fires around Madrid and Ávila forced some 100,000 evacuations and burned 55,000 hectares, the largest in Spain's recent history. On 7 August, a snapshot of six European cities put Madrid and Athens at or above the WHO daily guideline for fine particulate matter.

Two continents, one exposure, and in both cases it travelled a long way from where it started. You don't need a fire nearby to be breathing one.

What it does to the people on your panel

This is not a discomfort story. Wildfire PM2.5 is consistently associated with increased emergency department visits and hospitalisations for asthma and COPD, with relative risks that often exceed those of ordinary urban particulate matter, and elderly COPD patients carry the heaviest burden of hospitalisation and mortality risk.

The cardiac picture is just as clear. On days with dense smoke, one California analysis found elevated rates of emergency visits for myocardial infarction, ischaemic heart disease and heart failure, with the highest rates in adults aged 65 and over. After wildfires in Calgary, physician visits among older adults rose 11% for congestive heart failure and 19% for ischaemic heart disease. Fire-derived PM2.5 is thought to carry more cardiovascular toxicity than particulate matter from other sources.

Read that against a typical home health census, heart failure, COPD, over 75, indoors, often without air conditioning, and it stops being an environmental story and becomes a description of your caseload.

Why it stays invisible

Here is the part that should bother anyone responsible for a panel.

The exposure is regional, but the record is individual. Each patient's notes describe one person having a difficult week for no documented reason. Nowhere in the chart is there a field for the air outside was dangerous on Tuesday, and nobody fills one in.

The patient can't supply it either. She isn't checking an air quality index. She knows she felt worse, and she has probably already explained it to herself as her age, or a bad night, or nothing worth ringing about.

So the signal fragments. Forty patients in the same three counties deteriorate in the same seventy-two hours, and it arrives at the agency as forty unrelated anecdotes, most of which are never reported at all, because none of them individually felt like an emergency.

The only way that pattern becomes visible is if somebody asked all forty, in the same window, in the same way. Scattered symptoms with a shared cause look like noise until you have enough of them, close enough together, to see the shape.

The version that shows up in trials

The same exposure does something quieter and more expensive to a study.

Symptom scores, sleep quality, fatigue, exercise tolerance and respiratory endpoints all move when the air is bad, across whole regions, for days at a time. If that lands unevenly across sites or across an enrolment window, it enters the dataset as signal. It looks like a treatment effect, or it masks one.

It is a confounder that is genuinely knowable, smoke exposure is modelled at kilometre resolution, publicly, daily, and that almost nobody captures, because nothing in the standard visit schedule asks.

The point

Care systems are built to explain deterioration by looking inward: the medication, the disease, the adherence, the patient. That instinct is right most of the time, and this summer it was wrong across two continents at once.

Some of what happened to your patients in July had nothing to do with any of them individually. It happened to everyone in the postcode, on the same days, for a reason that was on the news and not in the notes.

You cannot chart the air. You can ask, consistently, and often enough to see when a lot of people start answering the same way at the same time, and then treat that as the population signal it is, rather than forty separate bad weeks.

Sources

• Copernicus Atmosphere Monitoring Service, Extreme wildfires in western Europe expose millions to dangerous air pollution levels, 2026 — https://atmosphere.copernicus.eu/extreme-wildfires-western-europe-expose-millions-dangerous-air-pollution-levels

• Crisis24, Extreme Heat and Wildfire Smoke Compound Public Health Risks in Europe, 2026 — https://www.crisis24.com/articles/extreme-heat-and-wildfire-smoke-compound-public-health-risks-in-europe

• Epidemiological and Clinical Evidence on the Association Between Wildfire PM2.5 and Pulmonary Health Outcomes — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12476873/

• Long-Term Wildfire Smoke Exposure and Increased Risk of Heart Failure in Older Adults, JACC, 2025 — https://pubmed.ncbi.nlm.nih.gov/40562508/

• Impact of Long-Term Cumulative Exposure to Wildfire Smoke PM2.5 on Cardiovascular Hospital Admissions Among Older Adults in the United States, JACC — https://www.jacc.org/doi/10.1016/j.jacc.2025.12.079

• The Cardiology Advisor, Wildfire Smoke, Health Effects, and the Relationship With Cardiovascular Disease — https://www.thecardiologyadvisor.com/features/wildfire-smoke-health-effects-relationship-cardiovascular-disease/