Indoor Air Quality, Allergies and Asthma

Allergies and asthma are individual conditions with a genetic and immune-system basis; indoor air quality doesn't cause them the way an infection causes a cold. What indoor conditions can do is trigger or worsen symptoms in people who are already susceptible, sometimes significantly. For someone with asthma, a trigger like PM2.5 or a strong fragrance can prompt an actual flare-up; for someone with allergies, an allergen like pollen or dust mite protein can set off the immune response that produces sneezing, congestion or itchy eyes.

Because susceptibility varies so much between people, two people in the same room can have completely different experiences: one might not notice anything, while another develops symptoms within minutes. This is worth keeping in mind throughout this guide; the goal isn't to eliminate every possible trigger from a home, but to identify the ones that matter for the people who actually live there and reduce exposure to those specifically.

These terms get used interchangeably, but they describe different things. An allergen is a substance, almost always a protein, that triggers an immune response in someone who has become sensitised to it: dust mite droppings, pollen grains, mould spores and pet dander are all common indoor allergens. A pollutant, in the air quality sense used throughout this Academy, is a measured substance like PM2.5, CO₂ or TVOCs, defined by a physical or chemical property rather than by whether it triggers an immune reaction.

The two overlap but aren't the same. Some allergens are large enough, or behave in a way, that means they partially show up in PM2.5 or PM10 readings; others don't register meaningfully at all. This distinction matters practically because it shapes what a consumer air quality monitor can and can't tell you.

Mould needs moisture to grow, and indoor humidity is the single biggest factor most households can control. Sustained relative humidity above roughly 60 to 70 percent, or localised condensation on cold surfaces like windows, creates conditions where mould can establish and spread, particularly in bathrooms, kitchens and poorly ventilated rooms.

Mould exposure is a well-established trigger for allergy symptoms and can worsen asthma in sensitive people. Because mould often grows in places that aren't immediately visible, behind furniture, under flooring, inside wall cavities, a persistent musty smell or repeated condensation is worth investigating even without visible mould. Keeping humidity in a healthy range, fixing leaks promptly and ventilating moisture-generating rooms are the most effective preventive steps.

Dust mites live in bedding, carpets, upholstered furniture and soft toys, feeding on shed skin cells. It's not the mites themselves but proteins in their faecal matter that trigger allergic reactions, and dust mite populations are strongly tied to humidity, thriving above roughly 50 percent relative humidity and struggling below it.

Pollen is seasonal and largely an outdoor source that enters homes through open windows, on clothing, or on pets that have been outside. Levels typically follow predictable seasonal patterns depending on the plants involved, tree, grass or weed pollen, and checking local pollen forecasts can help with timing decisions like when to keep windows closed.

Pet dander deserves particular attention because it behaves differently between species. Cat allergen in particular is carried on unusually small particles that stay airborne for long periods and spread easily through a home, even to rooms a cat rarely enters, which is part of why cat allergies can feel so persistent even in a house that looks and smells clean.

PM2.5, whether from cooking, candles, fireplaces, tobacco smoke or wildfire smoke drifting in from outside, is small enough to reach deep into the airways and is a well-documented trigger for asthma symptoms and general respiratory irritation. People with asthma are generally advised to pay closer attention to PM2.5 than the general population, since even moderate elevations can prompt symptoms that wouldn't affect someone without the condition.

Wildfire smoke deserves specific mention because it can elevate PM2.5 far beyond typical indoor sources, sometimes for days at a time. During smoke events, keeping windows closed and relying on filtration, rather than ventilating with polluted outdoor air, is the more protective approach.

Cleaning products, air fresheners, scented candles and personal care products release VOCs that can irritate the eyes, nose and throat in sensitive people, and strong fragrances specifically are a commonly reported asthma and migraine trigger, even though they aren't classified as allergens in the immune-response sense.

Choosing fragrance-free or lower-VOC products where practical, and ventilating well during and after cleaning, painting or using strongly scented products, reduces this category of trigger without requiring a complete change in household habits.

Reducing exposure to indoor triggers generally comes down to three complementary strategies, and knowing which one fits a given situation:

For most households managing allergy or asthma triggers, PM2.5, CO₂, TVOCs, temperature and humidity are the most useful measurements to track, alongside PM10 if pollen or dust is a particular concern. None of these can identify a specific allergen directly, as covered earlier, but they can reveal patterns: a PM2.5 spike during cooking, humidity creeping toward mould-friendly levels, or a stuffy, poorly ventilated bedroom.

It's worth being clear about the limits here. A consumer monitor is a tool for spotting trends and prompting practical changes, ventilating more, adjusting humidity, filtering during a smoke event, not a diagnostic device. It can't tell you whether a symptom is caused by pollen, dust mites, mould or something unrelated to air quality entirely. If symptoms are frequent, severe or not responding to the practical steps in this guide, that's a conversation for a healthcare professional, and existing asthma or allergy management plans should always take priority over general air quality guidance.