The mould comes back until the damp goes

You scrubbed the black spots off the shower seal, the window frame or the corner of the ceiling, and they came back. It is tempting to reach for a stronger bottle. People, and the public-health guidance, point somewhere else.

Mould comes back until the damp goes: clean it off, then fix the water that feeds it.

Why it comes back

Mould needs water. Some people say the thing that stops it is less water in the air — a window open, a dehumidifier — and two say cleaning is wasted if the leak, the failed seal or the damp behind it is still there. The US Environmental Protection Agency says the same in one sentence: if you clean up the mould but do not fix the water problem, the mould will most likely come back.

The EPA gives the numbers to aim at. Keep indoor humidity between 30 and 50 per cent where you can, and never above 60. Dry anything that gets wet — a spill, a leak, a soaked mat — within 24 to 48 hours, and in most cases mould will not grow. In a bathroom that means the fan or the window after every shower, and a quick wipe of the tiles and the seal. After any cleaning, one person adds, dry the surface properly, or you have just given the next patch a wet start.

What to clean it with depends on what it is growing on

This is where the arguments here start. Some people say bleach does not work on porous materials — caulk, wood, plasterboard — because it kills what is on the surface and leaves the growth inside, or breaks down before it gets there. Two say bleach works well on hard, non-porous surfaces like tile and glass. A mycologist here puts the two together: bleach is effective on a non-porous surface, or on a porous thing that can be fully soaked, and fails on dry porous material it cannot reach into. The problem is how it is used and what it is used on, not the bleach. The argument between the two sides, evenly split, comes down to that one question — how porous the surface is.

The health authorities give two slightly different answers for hard surfaces, so here are both. The US CDC says a bleach solution of no more than one cup of household bleach to a gallon of water, with the windows open — about one part bleach to sixteen of water. The EPA says hard surfaces can be scrubbed with detergent and water and dried completely. Either way, the scrubbing does the work: one person says soaking alone is not enough without a brush, and a brush, an old toothbrush or a cloth is what gets it off. One person points out that bleach is often used far stronger than it needs to be, which only makes more fumes. One caution says not to spray bleach as a fine mist, because breathing it in can injure your lungs; one person’s way of getting it onto a grout line or a seal is strips of toilet paper soaked in the diluted bleach, laid along the line.

White vinegar is what some people reach for instead, for grout and general mould. Others use hydrogen peroxide — the ordinary pharmacy-strength kind — which whitens as well. Both come with warnings: vinegar is an acid, so keep it off natural stone and marble, which it can stain or damage, one person warns; and hydrogen peroxide must never be drunk or kept in anything that looks like a drink — some people warn that swallowing it causes burns, another that the stronger grades on sale cause severe ones, and one person warns it can be mistaken for water. Which of these works best is a real argument with no winner, and what works on one person’s mould may not work on another’s. What decides it is partly what you are after: vinegar may be enough to kill live mould, while a stain left behind may need bleach, which one person found was the only thing that lifted it. The surface matters too, and whether the growth has gone into it.

Porous things are where cleaning runs out. The EPA says absorbent materials such as carpet and ceiling tiles may have to be thrown away once they are mouldy, because mould fills the spaces inside them and may be impossible to remove.

The shower seal

Mould in or under the silicone round a bath or shower has a plain answer from people who address it: if mould is growing under the caulk, the seal has failed and water is getting behind it. Cleaning the top will not reach it. The fix is to remove the old caulk, dry the area, and put new caulk on — and one person adds, on stone or marble, use a silicone made for stone, because the standard kind can stain it.

There is a recipe going round for a paste of bleach and baking soda to bring caulk back to white. Two things about it. People argue about whether bleach harms silicone and latex caulk at all, and do not settle it. And one person points out pure baking soda with bleach is generally safe, while another says the real risk is baking powder, which contains an acid, is easy to grab by mistake, and releases chlorine gas with bleach. When the safe answer costs nothing, take it: bleach and water only, or no bleach at all. There is a page here, Clean cheap, and never mix the bottles, on which household pairings are dangerous and why.

When it is not yours to clean

The EPA’s line is size. A patch under about 10 square feet — roughly three feet by three — you can usually handle yourself. Bigger than that, or after a lot of water damage, it points you to its guide for larger clean-ups; if the water was sewage or other contaminated water, call in a professional; and if you think mould is in your heating or air-conditioning system, do not run it, because it can spread mould through the building. Whoever cleans, the EPA says wear gloves and goggles and suggests an N95 mask, and one person here says the mask is not optional.

Some people may be better off not being the one doing it. The CDC says people with asthma or a mould allergy can react badly, and people with weakened immune systems or long-term lung disease can get lung infections from mould. England’s government guidance adds children, older people and pregnant women to those more at risk. If that is you or someone in your home, think twice about being the one who cleans it, and if anyone has symptoms — a stuffy nose, a sore throat, coughing or wheezing, sore eyes or a rash, which the CDC lists — it is worth asking a doctor.

If you rent, the damp may not be yours to fix either. The first question below is for you.

And if the mould you came about is inside a drinking bottle, a humidifier or the washing machine, the last question covers it.