Allergies and asthma at home: what the evidence actually supports

Allergy and asthma advice is full of confident recommendations, and the evidence behind them is more mixed than most product marketing admits. Some interventions have solid support, several popular ones have repeatedly failed to show benefit in trials, and the difference matters because the useless ones cost money and effort.

Here is a fair reading of where the evidence sits, and what a smart home can usefully contribute.

Identify the trigger before buying anything

This is the step that determines whether everything else helps, and it gets skipped.

House dust mite, pollen, pet allergen, mould spores and non-allergic irritants such as particulates and strong fragrances all cause similar symptoms and call for quite different responses. Reducing dust mites in a household whose actual trigger is a cat achieves nothing.

Allergy testing through a doctor — skin prick or specific blood tests — is the reliable way to know. Symptom timing is a decent clue in the meantime: worse in the morning and in bed suggests dust mite; seasonal and outdoor-linked suggests pollen; worse in a particular room suggests mould or something in that room.

Dust mites: what the evidence supports

Dust mites live in bedding, mattresses, carpets and soft furnishings, feed on shed skin, and produce the allergen in their droppings. They need humidity to survive, and that is the lever.

Humidity control has the clearest mechanism. Mites struggle to survive sustained relative humidity below about 50 per cent, because they absorb water from the air rather than drinking. Keeping bedroom humidity in the 40 to 50 per cent band is therefore a direct attack on the population rather than on the allergen after the fact.

Allergen-proof mattress and pillow covers have reasonable support, particularly as part of a combined approach. Trials of covers alone in adults have often been disappointing, which is worth knowing before spending heavily on them and nothing else.

Hot washing of bedding at 60 °C kills mites; cooler washes remove allergen but not the mites themselves.

Reducing soft furnishings in the bedroom — heavy curtains, deep carpet, an excess of cushions and soft toys — removes habitat. This is unfashionable advice and mechanically sound.

The honest summary from systematic reviews is that single interventions rarely produce dramatic improvement, while combined, sustained approaches do better. That argues for doing several cheap things consistently rather than one expensive thing.

Pollen: timing is the whole intervention

Pollen is outdoors, which makes it a ventilation-timing problem, and this is where automation is genuinely strong.

Pollen counts follow daily patterns. Grass pollen typically peaks in the early morning and again in the early evening; concentrations are lower after rain and higher on warm, dry, breezy days. Ventilating outside the peaks is free and effective.

Keep bedroom windows closed overnight during your season, which conflicts with the CO₂ advice elsewhere on this site — resolve it by ventilating hard at a low-pollen time and using the door rather than the window overnight.

Filter incoming air if you have mechanical ventilation, and check what filter grade it accepts.

Shower and change before the bedroom. Pollen arrives on hair, skin and clothes, and a bedroom kept sealed all day is undone by walking in from the garden.

An automation that checks a pollen forecast and closes windows or adjusts ventilation accordingly is one of the more clearly useful things you can build, provided the forecast covers your specific pollen type rather than a single index.

Air purifiers: effective, within limits

Purifiers with genuine HEPA filtration do remove airborne particles, and the evidence for symptom improvement is moderate — real, but smaller than the marketing implies. Understanding why makes the limits clear.

Dust mite allergen is mostly not airborne. It sits in bedding and carpet and becomes airborne briefly when disturbed, then settles quickly. A purifier cannot filter what is in your mattress.

Pollen and pet allergen behave better — they stay aloft longer, so filtration helps more.

Practical guidance if you buy one: size it by air changes per hour for the room rather than by a headline coverage figure, run it in the bedroom overnight where you spend the most consecutive hours, and choose a unit quiet enough at its useful setting that you leave it on. A large purifier on low is quieter and more effective than a small one at maximum.

Avoid ozone generators and be wary of ionisers, which can produce ozone as a by-product. Ozone is a respiratory irritant — the opposite of what you want.

Pets, honestly

Pet allergen is produced by skin, saliva and urine, is very small and sticky, stays airborne for a long time and persists in a home for months after an animal has gone.

The interventions with some support: keeping the animal out of the bedroom entirely, HEPA filtration, hard flooring rather than carpet, and frequent washing of pet bedding. Regular washing of the animal has modest and short-lived effect.

The claim that specific breeds are hypoallergenic is not supported by measurement; studies have found no consistent difference in allergen levels between so-called hypoallergenic breeds and others.

And the honest part: for a strongly sensitised person, no amount of filtration matches not having the animal in the bedroom, and a bedroom that has never had the pet in it is far easier than one being remediated.

What we would not spend money on

Ozone generators and ionisers, as above.

Sensors claiming to detect allergens, pollen or spores at domestic prices. They detect particles by size and infer, and they cannot tell you what those particles are.

Ultraviolet light in domestic ducts, where evidence of benefit for allergy is weak.

Scented purifiers or air fresheners, which add volatile compounds to air you are trying to clean.

Whole-house humidifiers bought reflexively, since raising humidity helps a dry throat and feeds dust mites simultaneously.

Cleaning, and the vacuum question

Cleaning matters, and how you clean matters more than how often.

A sealed vacuum with a HEPA filter is worth the money for an allergic household. The important word is sealed: a machine with an excellent filter and leaky joints simply blows fine dust out through the gaps, which is why some vacuums measurably raise airborne particulates while in use. Look for a claim about the whole system rather than the filter alone.

Damp dusting beats dry dusting. A dry cloth or a feather duster lifts allergen into the air and puts it back down elsewhere. A damp microfibre cloth captures it.

Someone else should vacuum if possible, or the allergic person should wear a mask and leave the room for twenty minutes afterwards while particles settle. Symptoms during and after cleaning are extremely common and entirely predictable.

Robot vacuums help by being frequent, which suits mite and pet allergen, but check they have a decent filter and empty the bin outdoors. A small robot with a poor filter run daily is not obviously an improvement.

An order of operations

1. Get tested, so you are treating the right trigger.
2. Measure bedroom humidity for two weeks. If it sits above 55 per cent and dust mite is your trigger, that is your first target.
3. Do the free things: bedding at 60 °C, fewer soft furnishings in the bedroom, pets out of it, shower before bed in pollen season.
4. Fix any damp, because mould and mites share the same cause.
5. Automate ventilation timing around pollen peaks and humidity readings.
6. Then consider a purifier, sized for the bedroom and quiet enough to leave running.
7. Review after a month against symptoms, not against sensor readings.

Step seven is the one that keeps you honest. The purpose is fewer symptoms, not better numbers on a dashboard.