Mold Allergies: Causes, Health Effects and How to Protect Your Home
Updated on April 18, 2026Dampness and mold are among the most common indoor air problems in the world. The World Health Organization estimates that 10 to 50% of indoor environments in Europe, North America, Australia, India and Japan are affected by dampness, and that people living in damp or moldy homes have roughly a 75% higher risk of respiratory symptoms and asthma. In the United States, a review of national studies by the Lawrence Berkeley National Laboratory puts the share of homes with damp or mold conditions at about 47%.
Behind these numbers lies a health issue that is easy to underestimate: mold is one of the main indoor air pollutants, and mold allergies are among the most common respiratory allergies. This guide explains what mold actually is, why it makes people ill, and the practical steps that reduce both the mold and your exposure to it.
What is mold?
A microscopic fungus
Mold is a living organism, more precisely a microscopic fungus that develops wherever moisture is present and triggers a chemical reaction on the surface it grows on. There are more than 80,000 varieties, active all year round. Four of them, found both outdoors and indoors, are responsible for most mold allergies:
- Alternaria: mostly outdoors, where it feeds on decaying plants, but also present in damp homes. Its colonies range from grey to black and look rather downy.
- Aspergillus: found almost everywhere. Depending on the species (there are close to 300), colonies range from white to grey through shades of yellow, green and blue. It grows in soil, compost and organic debris, but also in food, plasterboard, wood and cardboard, and in filtration and air-conditioning systems. It is particularly invasive.
- Cladosporium: the most abundant of all molds. Its brownish colonies range from dark olive green to black. Common in summer, it settles on windows, textiles, paint and plants.
- Penicillium: found in organic debris and decaying plants, and also the mold that grows on cheese rinds.
Mold is often given away by black or green patches and a musty smell, but it can also be invisible, for example when it grows inside a wall, which makes it hard to detect. Mold releases spores in large quantities. Spores are, in effect, the pollen of mold: particles that spread through the air and let the fungus proliferate. They are tiny, in the micrometre range, and travel easily through the air.
How spores spread through the air
To get into your home, spores hitch a ride on people, for instance by settling on clothing, and on every other available carrier, such as draughts and animals. Once inside, mold uses those same spores to spread from room to room. They develop quickly, especially in damp conditions, and are highly resistant. These spores are a genuine air pollutant and an allergen that can penetrate the airways and lungs, which is why mold is considered one of the main sources of indoor air pollution.
What causes mold in your home?
Mold needs moisture to grow, so it appears most often in rooms where water is used daily, such as the bathroom and kitchen, or in naturally damp spaces like cellars or near houseplants. Certain situations also encourage it:
- flooding
- leaks causing water infiltration through the roof, basement or pipes
- waterproofing problems that trap accumulated moisture
- inadequate ventilation, which prevents moisture from escaping and creates condensation
Mold is most often found on walls, drywall, furniture and wallpaper, which it gradually degrades. It can also stay hidden: the US Environmental Protection Agency (EPA) notes that mold frequently grows out of sight, behind wallpaper or drywall, above ceiling tiles, or inside walls around leaking or condensing pipes.
47%
of US homes have damp or mold conditions, in line with the WHO estimate of 10 to 50% of buildings worldwide.
Source: Lawrence Berkeley National Laboratory, review of US studies (incl. Mudarri and Fisk, Indoor Air, 2007)
Mold allergies: the effects on health
The health effects of mold fall into three categories.
The allergic risk is the most common. Inhaling mold can cause asthma and allergic rhinitis. The symptoms seen when an allergic reaction is triggered, sneezing, a runny or blocked nose, throat irritation and itchy eyes, come from a specific immune response with antibody production. The threshold of allergic reactivity varies from one person to another, and the severity of the reaction is not proportional to the dose of exposure. In the most severe cases, the allergy is associated with asthma, a condition that can be life-threatening.
The toxic or inflammatory risk relates to exposure to mold components (fungal spores, β-glucans and other parts of the mycelium cell wall) and to their metabolic products (mycotoxins, microbial VOCs). The effects vary in severity and can be acute or chronic. Allergic effects are not always easy to tell apart from so-called non-allergic inflammatory effects, which produce similar symptoms but involve a non-specific immune response.
The infectious risk from inhaling indoor mold, meaning the fungus multiplying inside the host, mainly concerns opportunistic fungal infections. These depend largely on the person's immune status, in particular immunosuppression linked to illness or medical treatment.
Mold can therefore harm your health in several ways. Because there are so many varieties, a mold allergy usually involves several of them at once, and it is often associated with other allergies, to dust mites, pollen, cats or dogs, especially in people with asthma. To reach a diagnosis, your doctor may ask which mold varieties are present in your home. One point is worth remembering: according to the EPA, dead mold can still cause allergic reactions, so it is not enough to kill it, it must also be removed.
Precautions to take when mold appears
How to react
If you have identified mold at home, first work out what is causing it: damp? A water leak? Treat the source of the problem before anything else. To remove the mold itself, if the affected area is small, less than about 1m² (roughly 10 square feet, the threshold the EPA uses), you can usually scrub it off with an all-purpose cleaner, diluted soap or detergent, then dry the area thoroughly, since mold thrives on moisture. Keep hard-to-clean objects away from the area. If the mold covers a larger surface, it is worth calling in a specialist in building moisture treatment.
Mistakes to avoid
When cleaning mold, take care not to add pollutants on top of pollutants:
- Avoid bleach. The EPA does not recommend chlorine bleach or other biocides as a routine practice during mold cleanup, and bleach is harsh on you and on your furniture.
- Do not use fungicides unless a specialist advises it.
- Avoid spreading spores through the air. For example, do not break up the material the mold is growing on, especially where contamination is heavy.
Never leave mold to settle in for the long term. Deal with it without delay.
Preventing mold: simple habits
A few habits go a long way toward avoiding nasty surprises:
- Fix leaks as soon as you can, and dry any water-damaged area within 24 to 48 hours, the window the EPA gives before mold usually starts to grow.
- Use an extractor hood in the kitchen and a fan in the bathroom.
- Keep indoor relative humidity in check. The EPA recommends staying below 50%, ideally between 30 and 50%, and never above 60%. A small humidity meter makes this easy to monitor.
- Keep your home clean and dry, air it regularly, and use a dehumidifier if needed.
Spores can survive extreme conditions, so stay vigilant.
Cleaner indoor air: where an air purifier fits in
Damp, mold and pollution all contribute to unhealthy air, which is a particular concern for vulnerable people, those with allergies, and young children whose lungs keep developing until around the age of seven.
An air purifier does not replace fixing the moisture problem, but it can reduce your exposure to what is already airborne. TEQOYA has taken this approach with air purifiers based on negative ions, which act on airborne particles, including mold spores and other allergens, through electrostatic settling. They use very little energy, are silent, and help limit the airborne movement of spores around the home. What a particle purifier does not do is remove the moisture or the mold source itself, so it works best alongside good ventilation and moisture control, not instead of them.
Worried about mold spores and allergens in your air?
TEQOYA filterless ionizers reduce airborne fine particles, allergens and mold spores, silently and with almost no energy use. Made in France.
The TEQOYA T200 suits bedrooms, while the TEQOYA T450 covers larger rooms.
Frequently Asked Questions
What are the symptoms of a mold allergy?
Mold allergy symptoms are mainly respiratory: sneezing, a runny or blocked nose, throat irritation and itchy, watering eyes. In people who already have asthma, mold can also trigger asthma attacks. Symptoms can appear immediately or with a delay, and their intensity varies a lot from one person to another.
How much mold exposure is harmful?
There is no single safe threshold. The severity of an allergic reaction is not proportional to the dose, so even limited exposure can affect a sensitive person, while others tolerate more. What matters most is your individual sensitivity and any existing conditions such as asthma or a weakened immune system.
Can I clean up mold myself?
For a small patch, under about 1m² (10 square feet), yes: scrub hard surfaces with detergent and water, then dry completely. For larger areas, heavy contamination, or mold linked to sewage or your ventilation system, the EPA advises calling a qualified professional.
Does an air purifier help with mold?
An air purifier reduces airborne mold spores and allergens, which can lower your exposure. It does not remove moisture or the mold growing on surfaces, so it complements, but never replaces, fixing the water problem and cleaning the affected areas.
Sources
- World Health Organization, "WHO Guidelines for Indoor Air Quality: Dampness and Mould", 2009. WHO
- Mudarri D., Fisk W.J., "Public health and economic impact of dampness and mold", Indoor Air, 2007; US prevalence data compiled by the Lawrence Berkeley National Laboratory. Berkeley Lab
- US Environmental Protection Agency (EPA), "A Brief Guide to Mold, Moisture and Your Home", updated 2026. US EPA
- Observatoire de la Qualité des Environnements Intérieurs (OQEI, ex-OQAI), « Campagne Nationale Logements », 2003-2005.
- ANSES, « Moisissures dans le bâti », 2016.
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