Ozone Levels and Headaches: The Summer Smog Connection
Ground-level ozone is not emitted by anything. It is manufactured in the air itself when sunlight drives a reaction between nitrogen oxides and volatile organic compounds, which is why it peaks on hot, sunny, windless summer afternoons and falls to near nothing overnight and through winter. The evidence linking ozone episodes to increased headache and respiratory symptoms is reasonably consistent, though the heat that produces ozone is itself a strong migraine trigger, which makes clean attribution genuinely hard.
Most people file ozone under "air pollution" alongside particulate and leave it there. That is a mistake, because ozone behaves almost nothing like fine particulate. Different source, different season, different time of day, and a different geography — often worse in the suburbs and countryside downwind of a city than in the city centre itself.
If you have a summer headache pattern that does not match your pressure log, ozone is one of the more plausible explanations, and one you can do something about.
A pollutant nobody emits
Ozone in the stratosphere, tens of kilometres up, is the layer that absorbs ultraviolet radiation. That ozone is useful and its depletion is a separate problem entirely. Ozone at ground level, the kind you breathe, is a pollutant. Confusing the two has generated decades of muddled public understanding.
Ground-level ozone is a secondary pollutant. No chimney, exhaust pipe or factory releases it. It is created in the atmosphere from two families of precursor gases:
- Nitrogen oxides, mostly from combustion — vehicles, power generation, industry
- Volatile organic compounds, from fuel evaporation, solvents, paints, industrial processes and vegetation, which emits large quantities of reactive organic compounds naturally
Add sunlight, and photochemistry does the rest. Ultraviolet light splits nitrogen dioxide, the fragments react through a chain involving the organic compounds, and ozone accumulates. The reaction takes hours, which is the single most important fact about ozone's behaviour, because it means the ozone is not made where the precursors are released.
This is why ozone is often worse outside cities than in them. Precursors are emitted in the urban core during the morning rush. The air mass carrying them drifts downwind through the day while the chemistry proceeds. By mid-afternoon the ozone maximum has moved tens of kilometres out, into suburbs, exurbs and open country. There is a second reason the city centre reads low: fresh nitric oxide from traffic actively destroys ozone in a reaction called titration, so the most polluted-looking street can have the lowest ozone reading on the network. People who move out of a city for cleaner air sometimes find their summer symptoms get worse. That is not imagination.
The daily and seasonal shape
Ozone has the most predictable temporal profile of any common pollutant.
Through the day: near zero before dawn, rising after sunrise as photochemistry starts, climbing through late morning, peaking somewhere between mid-afternoon and early evening, then collapsing after sunset as production stops and deposition and titration remove what is left. The overnight minimum is genuinely low.
Through the year: a strong summer maximum and a winter minimum close to zero in most temperate locations. Winter sunlight is too weak and the angle too low to drive the chemistry at any useful rate.
Compare this to PM2.5, which does the opposite. Fine particulate in most temperate cities peaks in winter, when heating emissions are high, inversions are strong and the boundary layer is shallow. It is often lowest on a hot summer afternoon when the mixing depth is greatest. Two pollutants, opposite seasons, opposite times of day.
That contrast is useful diagnostically. If your headache pattern is a summer-afternoon phenomenon, particulate is an unlikely explanation and ozone is a plausible one. A still winter morning pattern points the other way. Anyone relying on a single "air quality index" number misses this entirely, because the index reports whichever pollutant is worst that day without telling you the two behave completely differently.
What ozone does in the airway
Ozone is a powerful oxidant. That is the entire mechanism, and it explains most of what follows.
Direct airway irritation. Ozone reacts with the lipid and protein layer lining the respiratory tract, generating reactive products that irritate nerve endings and damage epithelial cells. The symptoms are familiar to anyone caught outdoors during a bad episode — throat irritation, a cough, chest tightness, and a sensation that a full deep breath is somehow unavailable. Lung function measurably declines during exposure in healthy adults, not only in people with asthma.
Airway inflammation. Beyond the immediate irritation, ozone provokes an inflammatory response that develops over hours and persists after the exposure ends. Airway responsiveness increases, which is why ozone episodes are associated with asthma exacerbations and increased respiratory presentations.
The route to headache. Two plausible paths. The first is inflammatory, the same general mechanism proposed for fine particulate — airway inflammation contributing to systemic inflammatory signalling, which lowers the migraine threshold in a susceptible person. The second is more direct: ozone irritates trigeminal nerve endings in the upper airway, and the trigeminal system is the one implicated in migraine pain. Neither is established, but both fit what is known.
Exertion multiplies everything. Ozone effects scale steeply with the volume of air moved through the lungs, and heavy exercise shifts breathing to the mouth, removing what limited scrubbing the nasal passages provide. This is why the standard advice on ozone days concerns exertion rather than simply being outside.
The heat confound, stated honestly
Here is where anyone claiming certainty is overreaching.
Ozone episodes require heat and strong sunshine. Heat is itself one of the better-established environmental migraine triggers, acting through dehydration, sleep disruption, vasodilation and general physiological strain. High ozone days are hot days almost by definition. The two exposures arrive together, in the same air mass, at the same hour of the same afternoon.
Studies that attempt to separate them statistically exist, and some find an ozone association that survives adjustment for temperature. Others do not. The honest summary is that ozone plausibly contributes independently, that heat is definitely contributing, and that on any given afternoon you cannot tell from your own experience which one got you.
This matters practically, because the responses overlap but are not identical. Staying indoors in cooled air addresses both. Moving your run to early morning addresses both, since ozone and temperature are at their daily minimum then. Hydration addresses heat but does nothing for ozone. HEPA filtration addresses fine particulate but is largely useless against ozone, which is a gas and passes straight through.
There is a third variable riding along. The synoptic pattern that produces an ozone episode is a blocking high — a persistent ridge or heat dome that parks over a region, suppresses cloud, kills the wind and stops the air mass moving on. Under that pattern the barometer is high and, crucially, flat. Day after day of no change.
So the picture on a bad ozone day is: hot, sunny, still, high steady pressure, elevated ozone, and often several consecutive days of the same. If you feel unwell and reach for your pressure log, you will find nothing there, because the pressure genuinely is not doing anything. That absence is itself the clue. A heat dome over somewhere like Phoenix produces exactly this signature, and the same pattern appears in inland valleys and on the downwind fringes of large metropolitan areas wherever the summer sun is strong.
Timing: the single most useful piece of advice
Because ozone follows the sun with a lag, the timing of outdoor activity changes your dose more than almost anything else you could do.
Morning is the good window. Concentrations before about ten in the morning are typically a fraction of the afternoon peak, and temperature is at its lowest. Shifting a run, a ride or garden work from five in the afternoon to seven in the morning cuts your dose substantially with no change to the activity itself.
Late afternoon and early evening is the bad window. This is the daily maximum, and it is also, unhelpfully, when most people are free to exercise. Restructuring the week around that is worth the inconvenience if you have identified ozone as a trigger.
Evening after sunset is reasonable again. Production stops with the light and concentrations decay. Not as clean as morning, but much better than the peak.
Route choice matters less than hour choice. On a high-ozone afternoon a traffic corridor may counterintuitively read lower for ozone than open country, though it will be worse for nitrogen dioxide and particulate. There is no free option. Change the hour, not the route.
Practical exposure reduction
- Check the forecast, not just the current reading. Most agencies issue next-day ozone forecasts in summer, and because the chemistry depends on predictable meteorology those forecasts are reasonably reliable. Knowing tomorrow is an ozone day lets you move the run in advance.
- Keep windows shut during the afternoon peak and ventilate overnight. This is the opposite of the advice for particulate episodes, which is one more reason to know which pollutant you are dealing with. Indoor ozone is a fraction of outdoor levels because it reacts with surfaces as soon as it enters.
- Do not run an ozone-generating air cleaner. Devices sold as ionisers or air sanitisers that produce ozone deliberately are counterproductive for anyone with a headache or respiratory problem.
- Activated carbon, not HEPA, if you want filtration that touches ozone at all. Carbon saturates over time and needs replacing.
- Treat the heat separately. Hydration, cooling, shade and avoiding the hottest hours are worth doing on their own terms, whatever the ozone reading.
Telling ozone apart from your other triggers
Five columns, once a day, through a summer: headache and severity, maximum temperature, ozone reading, PM2.5 reading, pressure trend.
Patterns to look for. Summer-afternoon onset, several consecutive days during a heat spell, no pressure movement and an ozone reading in the upper part of its local range points toward the ozone-and-heat complex. Cold, still, hazy winter days with high flat pressure and elevated PM2.5 point toward particulate. Active weather, wind and a sharply moving barometer in any season point toward pressure.
Separating ozone from heat within that first group is the hard part, and it may not be possible from your own data. The partial signal available is the comparison between hot days with low ozone — a windy one, or an overcast humid one — and hot days with high ozone. If the low-ozone hot days are noticeably better, that is suggestive rather than conclusive, and there are rarely many such days in one summer. Comparing a marine-influenced location such as Seattle with an inland one during the same heat event can be informative for the same reason.
Frequently asked questions
Why is ozone higher in the suburbs than in the city centre?
Because the chemistry takes hours, so the air mass carrying the precursors has drifted downwind by the time the ozone has formed. Fresh nitric oxide from dense urban traffic also destroys ozone locally, keeping city-centre readings low. Downwind suburbs get the worst of it.
Does ozone cause migraine or just irritation?
The immediate effect is respiratory irritation. The route to migraine is indirect, through airway inflammation and possibly trigeminal irritation lowering the attack threshold. The association in population studies is modest and difficult to separate from concurrent heat.
When is the safest time to exercise outdoors on an ozone day?
Early morning, before about ten, when ozone and temperature are both near their daily minimum. Late afternoon and early evening is the worst window, which is unfortunately when most people train.
Will an air purifier protect me from ozone?
A HEPA filter will not, because ozone is a gas. Activated carbon adsorbs some but has a finite capacity. The most effective indoor measure is keeping windows shut during the afternoon peak, since ozone decays rapidly once it enters a building.
Why does my air quality app say the air is fine on a bad ozone day?
Check which pollutant the index is reporting. Some default to particulate, which is often genuinely low on a hot sunny afternoon when mixing is deep. Look for a separate ozone figure rather than trusting one headline number.
Is ozone worse in a heatwave?
Yes, for compounding reasons — more sunlight, faster reaction rates, stagnant air under the blocking high, and increased emissions from heat-stressed vegetation. Heatwave and ozone episode are close to the same event, which is precisely why attribution is so awkward.
My barometer is flat but I feel dreadful. What is going on?
That is the classic signature of a stagnant high. Pressure is doing nothing, but the same pattern concentrates ozone in summer and particulate in winter while imposing heat stress. A flat barometer during a hot spell should send you to the air quality reading, not to the conclusion that there is no weather explanation.
Where this leaves you
Ozone is the summer counterpart to winter particulate — opposite season, opposite time of day, opposite geography. It is manufactured by sunlight out of traffic and industrial emissions, it peaks in the afternoon downwind of where the pollution was released, and it arrives inseparably bundled with the heat that made it.
You will not be able to prove which half of that bundle is causing your headache. You can still act on the cheap parts: move outdoor exertion to the morning, keep the windows shut through the afternoon peak, check the ozone forecast the day before in summer, and stop expecting your pressure log to explain a heat dome.
Pressure Pal tracks barometric pressure and its trend alongside your logged symptoms, which is what lets you recognise a flat-barometer heat and smog episode for what it is instead of hunting for a pressure change that was never there.