Mold: prevention, control and expert tips

Susanne Loacker

Mold in a house or apartment is dangerous. Find out here why this is the case, how it occurs, how you can prevent it, and what you can do to stop it from spreading.

The picture shows a wall that is heavily infested with mold.
© Getty Images / Ekspansio

In this article

What makes mold so dangerous?

Mold spores in the house are toxic and dangerous for humans. They can lead to respiratory diseases and allergies. In infants, they can triple the risk of developing asthma and allergies, according to a Finnish study. But what makes mold so dangerous? And what are the differences between the various sorts of mold?

The following applies to mold: the dose makes the poison. We encounter mold spores everywhere – in rotting leaves or in garbage containers, for example. They only become dangerous in higher concentrations and with insufficient air circulation. This is because some mold spores can settle in the lungs and lead to severe respiratory diseases. Therefore, at the first sign of mold, don’t wait too long, but remove it immediately.

What types of mold are there in a living room?

According to experts, there are well over 250,000 different types of mold, but in the home we usually find just two sorts: green and black mold. Both species are dangerous for humans – but can be quickly identified. Black mold leaves significant black mold spots on the walls, green mold usually forms in damp rooms, for example in the joints of bathroom tiles. Yellow mold in upholstered furniture and red mold in paper wallpaper are quite rare in this country. White mold also occurs in living rooms – the tricky thing is that it is usually discovered quite late. The typical musty smell of a damp cellar can be an indication of white mold. But how does mold actually develop in the house?

Mold spores in the house: how and where does mold develop?

Since mold spores are transported through the air, they are practically omnipresent in buildings. As with any fundamentally unhealthy substance, the dose makes the poison, as they say.

Whether or not mold grows in a house or apartment depends a great deal on the behavior of the residents – but also on the structural conditions. The most common causes of mold in the house are:

  • Incorrect ventilation or heating
  • Construction defects
  • Cracks in the masonry
  • Moisture rising from the cellar
  • Damage to water pipes

In new buildings, mold can form if the correct drying time for the building materials has not been observed and moisture is subsequently released. For this reason, it is always advisable for an expert to check each individual construction phase in a new building.

Condensation water also favors mold. That’s why bathrooms are often infested by the harmful fungus, because a lot of water vapor is produced in the shower. Mold is also frequently found in bedrooms, as the body’s perspiration at night creates high levels of humidity in the room.

The first indication of a mold infestation is a musty, damp odor. A stronger infestation can be identified by typical dark spots on the walls. But how do you get rid of the fungus? Or even better: how can you prevent the infestation in time? The most important measures are described below.

The picture shows initial mold stains forming on a bathroom ceiling.

Treat mold correctly right from the start: there is still time to remove mold stains in the bathroom yourself.

Preventing mold: ventilate and heat properly

The easiest way to keep your home free of mold is to ensure proper heating and ventilation. This is because our breath alone increases the level of moisture in the air of houses and apartments. All it takes is some cooking fumes from the kitchen or steam from the bathroom, and the air in a room can quickly exceed the ideal humidity of 40 to 60 percent. That’s when air exchange is necessary: supersaturated humid air must go out, while fresh, drier air must come in.

In the summer you can air room more often and for longer, whereas in winter you should limit yourself to shock ventilation: open the window completely and let in the cold air from outside for five to ten minutes. Ideally, you can open opposite windows at the same time to create a draft that will replace the air even faster. Repeat this process three to four times a day. It’s important not to air rooms for too long so that the masonry doesn’t cool down too much. This in turn can lead to mold growth.

The importance of switching off the heating when airing the rooms is self-explanatory. Before and after ventilation, however, the heating should remain switched on, at least at a low level, throughout the winter. In the coldest season, it is recommended to heat the living room to no less than 16 degrees at night – this corresponds to a level 2 heating setting.

It may be worth installing an additional dehumidifier in bathrooms and utility rooms. This removes additional moisture from the air if regular airing is not sufficient. Even more information on the correct heating and ventilation of bathrooms can be found in our article “Heating a bathroom”.

The basement should also be aired regularly in the winter. If your basement is insulated, it’s also worth considering installing a heating system. This will prevent the basement – and the rooms above it – from cooling down too much. Read our article “Heating a basement” to find out how to heat and air your basement properly.

Tip: in the summer, it’s better to keep the basement windows closed, otherwise condensation will form on the walls, which can contribute to the formation of mold.

Combating mold

Anyone who finds regularly recurring mold or infestations covering a large area should contact a specialist company. The professionals will not only remove the mold, but also check whether structural defects are causing mold growth. To combat local infestation and surface mold, for example in bathroom joints, remedies from the pharmacy and home improvement store – or household remedies that everyone has at home anyway – can help.


Wipe superficial mold, such as traces found on walls or windows, with a damp cloth. Then apply methylated spirit (ethyl alcohol) or isopropyl alcohol. Spraying on the alcohol is not advisable, as it will evaporate before it reaches the wall. The alcohol disinfects the area, killing all organisms – including mold. Alcohol is also the solution recommended by mold expert Elian Bertschi (see below).

Chlorine or hydrogen peroxide

A similar effect can be achieved with cleaning products containing chlorine or hydrogen peroxide. However, you should only use these products with a respiratory mask and in rooms that can be well ventilated because the vapors are poisonous. They also leave unsightly bleach stains on fabrics, so you should wear protective clothing and protect the furniture. In addition, the substances are bad for the environment. So it remains questionable whether these products should be your remedies of choice – and our expert advises against them (see below).

Tea tree oil

Tea tree oil from the drugstore is also effective against mold. Prepare a mixture of tea tree oil and water (40 milliliters of tea tree oil to one liter of water). One tablespoon of liquid soap in the mixture helps the oil to combine with the water. Use a sponge or spray bottle to apply the mixture to the affected areas. This household remedy is very effective for fighting mold and mold spores but has a very strong odor that takes a few days to fade.

Baking soda

Baking soda mixed with water to form a thick paste is also a proven and inexpensive remedy against mold. Spread it on, leave it to take effect overnight, then wipe it off. Its antibacterial effect is a reliable way to kill mold and its spores. You should be careful with marble tiles, however, as the baking soda attacks the material. Even on stoneware tiles that are not painted and therefore open-pored, it is not advisable to work with baking soda.


Mold is a living being. This means it doesn’t like temperatures below freezing point or intense heat. You can’t exactly put a wall in the freezer overnight, but a hairdryer can be used to combat mold – the hotter, the better. A hot air dryer from the hardware store will be more effective than the small travel hair dryer you have in your drawer. Repeat the treatment regularly and for at least –four to five minutes, even if no new mold is visible in the area.

Minerals and acidic paint

Either a borax solution (from the mineral borax, also known as tincal) or alkaline paint, i.e. paint with a high pH value, will help in the long term. This can be silicate paint, lime paint or mineral paint. However, before you buy a product, make sure that no organic resins are contained in it. These types of resins are often used as binders in emulsion paints; dispersion paint is not suitable for areas prone to mold growth. If the previous coat consisted of dispersion paint, sand down the surface before repainting.

The picture shows a man working with a mold spray. He is wearing a protective mask.

If you decide to combat mold yourself with chemical products, you must protect your respiratory tract.

Elian Bertschi is deputy managing director of Dobler Bautenschutz, a company specialized in mold protection. The director of the company, Franz Dobler, is a court-approved mold expert.

How common is mold in Swiss houses and apartments?

“Common. Very common.”

What are the most frequent causes?

“Buildings are often built too fast, which means that the materials are unable to dry out properly. However, subsequent water damage and incorrect furnishing are also quite often to blame – for example if people place large pieces of furniture too close to outside walls.”

Is there a general rule about how much mold you can treat yourself before you should call in a professional?

“I would say: up to the palm of one hand.”

What is your preferred method for smaller areas like this?

“High-proof ethanol. Many people use bleach (i.e. chlorine, editor’s note) – and are not aware that in doing so, they are bringing more poison into their apartment or house.”

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