Indoor Air Pollution: The Triggers Inside Your Home
Indoor air is where the exposure actually happens. People in temperate climates spend something close to ninety per cent of their lives inside buildings, so total personal exposure to almost any airborne agent is dominated by indoor concentrations rather than the outdoor figure your air quality app reports. And indoor air is not simply filtered outdoor air: it is outdoor air plus everything the building emits, everything the occupants do, and however much or little the ventilation removes. For a headache-prone person the biggest single controllable source in most homes is unvented gas cooking, followed by inadequate ventilation and damp.
The outdoor number is what gets published, monitored and forecast. The indoor number is what you breathe. In a home with a gas hob and a closed kitchen door, the week's peak nitrogen dioxide concentration can occur while somebody makes an omelette, and it will be higher than anything the nearest street monitor recorded.
This is not an argument that outdoor pollution does not matter. It is an argument that if you have been diligently checking a regional index while cooking on gas in an unventilated kitchen, you have been watching the wrong meter.
Why buildings concentrate what happens in them
Two properties of an enclosed space do all the work.
Small volume. A kitchen might hold 30 or 40 cubic metres of air. A moderate source releases into that rather than into the atmosphere, so concentrations climb quickly.
Slow air exchange. Modern buildings are deliberately airtight, because leakage is wasted energy. Older draughty houses replaced their entire air volume several times an hour without anyone doing anything. A well-sealed contemporary home may manage less than half a time an hour if nobody opens a window or runs a fan.
The consequence is a rule that applies to every source below: concentration is emission divided by ventilation. Reduce the emission or increase the ventilation, and both work. Filtering a source you have not addressed does not, because most filters handle particles while many indoor problems are gases or moisture.
The gas hob, which is probably your largest single source
Burning gas indoors without extraction is the closest thing to an unambiguous finding in indoor air quality.
A gas hob produces nitrogen dioxide, carbon monoxide, formaldehyde and ultrafine particles directly into the room. Measurements in ordinary kitchens routinely find nitrogen dioxide peaks during cooking that exceed outdoor hourly guideline levels. Gas ovens, enclosed and hotter for longer, are worse per meal than hobs. Unvented gas or paraffin heaters are worse still and should not be used in occupied rooms.
Nitrogen dioxide is an airway irritant acting on trigeminal nerve endings in the upper airway, and the association between gas cooking and childhood asthma is among the better-supported findings in this field. The specific migraine link is not established, but an irritant gas at concentrations that provoke measurable airway inflammation is a reasonable suspect for anyone whose head pain follows evening kitchen time.
Carbon monoxide deserves its own line. Headache is the classic first symptom of carbon monoxide exposure, and a poorly maintained appliance, blocked flue or faulty boiler can produce concentrations causing recurrent headache without ever reaching a level that produces collapse. Any headache that improves when you leave the house and returns when you come back, particularly if other household members or pets are affected, should prompt an alarm and a boiler service before anything else on this page. This is the one indoor air problem that can be lethal, and it is cheap to rule out.
What to do. Use the extractor hood every single time, from before you start to several minutes after you finish, and check that it vents outside rather than recirculating through a charcoal pad. Use the back burners, which most hoods capture better. If you are replacing a hob anyway, induction removes the source entirely.
Cooking particulate, which is not a gas problem
Even on an electric or induction hob, cooking is a major particle source. Frying, searing, grilling, toasting and anything that browns food produces large quantities of fine and ultrafine particulate. A single session of high-heat frying can lift kitchen PM2.5 to levels comparable with a bad outdoor pollution day.
Because this is particulate rather than gas, it responds well to both extraction and filtration. A HEPA purifier in an open-plan kitchen will visibly pull the post-cooking peak down over half an hour, which is one of the few situations where a consumer monitor gives immediate, satisfying feedback.
Two related sources are consistently underestimated. Candles are open combustion in a small room, more so with scented wax that also adds volatile organic compounds. Wood burning stoves leak particulate into the room every time the door opens for refuelling.
Stuffiness, carbon dioxide, and the headache in the meeting room
Everybody has experienced the specific dull-headed, foggy, mildly headachy feeling of a full room with the door shut. Occupants exhale carbon dioxide continuously, and in a poorly ventilated space it accumulates.
Outdoor carbon dioxide sits around 420 parts per million. A well-ventilated room stays under about 800. A bedroom with two adults and a closed door overnight commonly exceeds 1,500 and can pass 2,500.
What the evidence supports, carefully. Controlled studies have found decrements in cognitive performance and increases in reported headache, fatigue and poor concentration at elevated indoor carbon dioxide levels. Whether the gas is the causal agent at these concentrations, or simply an excellent proxy for poor ventilation and therefore for everything else accumulating alongside it, remains genuinely debated.
For practical purposes the distinction hardly matters. Carbon dioxide is cheap to measure and correlates tightly with ventilation rate, making it the best available single indicator of whether a room is adequately ventilated. A monitor reading 2,000 in your bedroom each morning tells you something useful regardless of which molecule is responsible.
The bedroom case is the most relevant one for migraine, because it is eight continuous hours and because sleep quality is one of the strongest determinants of headache frequency. A trickle vent open, a door ajar, or a window on the smallest latch setting usually takes an overnight bedroom from stuffy to acceptable.
Damp, mould and the humidity band
Indoor relative humidity has a comfortable middle and two unpleasant ends.
Too damp, above roughly 60 per cent sustained, and you get condensation on cold surfaces, mould growth, dust mite proliferation and a rising allergen load. Mould exposure is associated with upper airway irritation, congestion and sinus symptoms frequently confused with sinus headache that are often actually migraine with prominent nasal features.
Too dry, below about 30 per cent, the standard winter condition in a heated home in a cold climate, and the nasal mucosa dries out. That produces its own congestion, nosebleeds, throat irritation and a sensation of sinus pressure.
The workable target is the 40 to 55 per cent band. A cheap hygrometer in the bedroom and in the coldest room tells you which end of the problem you have.
Damp is often a ventilation problem wearing a disguise. Every occupant adds moisture by breathing, showering, drying laundry and cooking. In an airtight home with nothing extracting that moisture, it condenses on the coldest wall. Extractor fans in bathrooms and kitchens are the fix, and drying laundry indoors without ventilation is the single most reliable way to create a damp problem. This is a particularly common pattern in the older, retrofitted housing stock of cities like London, where insulation improvements sealed buildings that were never given corresponding ventilation.
Dust, dander and the bedroom
The bedroom is worth treating separately because of how much time you spend in it and because the surfaces are all soft.
Dust mite allergen accumulates in mattresses, pillows and bedding, and mite populations thrive above about 50 per cent humidity. Pet dander is persistent and remains in a home for months after a pet has gone. Neither causes migraine directly, but allergic rhinitis is a recognised comorbidity that increases headache burden through chronic congestion and poor sleep.
Measures with reasonable support: wash bedding weekly at high temperature, use protective covers if allergy is confirmed, keep humidity in the target band, and keep pets out of the bedroom. Removing carpet is a bigger intervention with more modest returns.
Fragrance and the products you brought in
Scented candles, plug-in air fresheners, aerosol sprays, perfumed cleaning products and laundry scent beads all deliberately release volatile organic compounds into your home. So do new furniture, fresh paint and new carpet, and those deserve their own treatment: the mechanisms and time courses of solvent and formaldehyde emission are covered in the companion post on volatile organic compounds and headaches.
Two points about fragrance specifically. Osmophobia, a heightened aversion to smells, is a recognised migraine feature and strong fragrance is among the most commonly self-reported triggers in patient surveys. And air fresheners do not clean air. They add compounds to it, some of which react with indoor ozone to form formaldehyde and secondary particulate. Switching to unscented versions of everything is a cheap and reversible experiment.
Ventilation, filtration, and knowing which one you need
The two interventions solve different problems and are not interchangeable.
Filtration removes particles. A correctly sized HEPA unit handles cooking particulate, smoke that has entered the house, pollen, dander and dust. It does essentially nothing for nitrogen dioxide, carbon dioxide, formaldehyde or moisture.
Ventilation removes everything, including gases and water vapour. It is the only tool that works on carbon dioxide and the main tool for moisture. Its limitation is obvious: it imports whatever is outside, which is why the advice reverses during a smoke episode or a bad ozone afternoon.
Activated carbon sits between them, adsorbing some gases and odours, with a finite capacity requiring replacement.
The sensible sequence: vent or remove the source first, ventilate second, filter third. Buying a purifier while continuing to cook on an unvented gas hob is the wrong problem in the wrong order.
Measuring it without buying a laboratory
You do not need much. A carbon dioxide monitor with a true optical sensor gives an honest ventilation reading and is the most informative device for the money. A hygrometer costs almost nothing and settles the damp question. A consumer particulate monitor is useful for seeing what cooking does to your rooms, provided you treat the absolute numbers as approximate and watch the shape of the curve instead. Devices claiming to measure total volatile organic compounds are rough relative indicators, not instruments. A carbon monoxide alarm is a safety device rather than an air quality one, and every home with combustion appliances should have it.
A note on pressure, because the two interact
Indoor air is not sealed off from outdoor weather. A stagnant high with a temperature inversion concentrates outdoor pollutants and simultaneously reduces the wind-driven infiltration that ventilates buildings, so both degrade together. Cold, still weather is also when people are least likely to open a window.
The result is a familiar pattern: a stretch of flat, high barometric pressure, a run of headaches, and no pressure change to explain it. A migraine tracker app such as Pressure Pal that records the pressure trend beside your entries lets you separate a bad run on a moving barometer, which points at pressure, from a bad run on a flat one, which points at whatever is accumulating in the air around you.
Frequently asked questions
Is indoor air really worse than outdoor air?
Often, for specific pollutants. Indoor concentrations of formaldehyde, volatile organic compounds and, in gas-cooking homes, nitrogen dioxide commonly exceed outdoor levels. Outdoor air is usually better for ozone, which decays rapidly indoors.
What is the single most useful change for a headache-prone person?
Ventilate the kitchen properly every time you cook, especially with gas. After that, get the bedroom ventilated overnight and the humidity into the 40 to 55 per cent band.
Could my headaches be carbon monoxide?
It is uncommon but serious enough to rule out. The pattern to watch for is headache that improves away from home, affects more than one household member, and coincides with heating season. Fit an alarm and have combustion appliances serviced.
Is opening a window always the right answer?
No. It is the default, and it reverses during wildfire smoke, a high-ozone afternoon, a pollen peak if you are allergic, or beside a busy road during the rush hour. Ventilate at the times when outdoor air is at its best.
Does a HEPA purifier help with gas cooking emissions?
It removes the cooking particulate but not the nitrogen dioxide, which is a gas. Extraction to outside is the only reliable answer for the gas fraction.
How do I tell an indoor trigger from a weather trigger?
Look at where you were. A pattern that improves within hours of leaving the building, or that disappears entirely on holiday and returns on the first evening home, points indoors. A pattern that follows the barometer regardless of location points at weather.
Where this leaves you
Indoor air is the air you actually breathe, and unlike the weather, most of it is under your control. The interventions that matter are dull and mechanical rather than technological: extract the cooking, vent the bathroom, dry the laundry somewhere with airflow, crack the bedroom window overnight, keep humidity in the middle band, check the carbon monoxide alarm.
Fix sources and ventilation first. Reach for a filter afterwards, for the particles that remain.
Pressure Pal tracks barometric pressure and its trend beside your logged symptoms, which is what separates a run of bad days driven by moving weather from a run driven by still air and whatever is accumulating in it.