The Texas towns and cities inundated by Hurricane Harvey’s torrential downpours are finally drying out, but the storm left a menace behind: mould.

Just 24 hours after a heavy rain, as the Centers for Disease Control and Prevention and now many Texans can attest, the fungi can begin to grow and invade a home.

“Mould appeared almost instantly,” said Alan Tillotson of Cypress, a community about 40 kilometres northwest of downtown Houston. He and his wife, Vicki, returned to their neighbourhood after having fled Harvey for most of last week. They started pulling off drywall and found big black circles of fuzz behind it. “We had it growing visibly on the materials we were removing. The goal is to get all the wet stuff out before it becomes a science experiment.”

Blossoming on walls, on furniture, on clothes and potentially in every crevice and corner of every soaked property, mould not only damages homes and businesses but affects human health. Exposure can trigger a stuffy nose, irritated eyes, cough or respiratory problems.

Mould is so ubiquitous because it reproduces and spreads via pollen-like spores that are lightweight and travel easily through the air, thus exposing people through inhalation and skin contact. Complicating the situation: those spores can last a long time.

Indeed, all mould needs to survive and thrive is moisture, oxygen, a surface to grow on and a food source. Moulds feed on dead, moist organic matter, including leaves, wood, cloth, paper, even dust. Smaller than the head of a pin, spores can hang in the air for hours — where they can be inhaled or ingested.

Dead spores still contain allergens and so can affect health, which is why it’s not enough to kill mould. It must be removed.

Tillotson said he and his wife have suffered no ill effects so far. They quickly ventilated rooms, opened windows and turned on fans. They have already had the drywall stripped down to the studs and their hardwood floors removed.

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“The water came in so fast,” he recounted Tuesday. “We had no idea how high it would go. If I had to do it all over again, I’d have already moved away.”

Methods to remove mould run the gamut. There’s simple, as in scrubbing with a fungicide mixed with bleach and water. There’s also complicated, which can involve wet vacuuming or vacuuming with a high-efficiency air purifier.

Yet mould expert Nick Gromicko, founder of the National Association of Certified Home Inspectors, thinks some victims of Hurricane Harvey may never be able to remove all the mould.

“Mould can be hidden behind walls where spores are not detected, and mould can grow very rapidly within days,” he said. “Mould is literally a monster that grows like that vintage movie The Blob ... It doesn’t die. If it has food and water, it will live forever.”

With thousands of homes and businesses along the Texas Gulf Coast unreachable for days — and perhaps uninhabitable for weeks or months — Gromicko said many structures likely can’t be rescued.

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“Where homes have been in standing water for so long, the cure for most of these mould problems is going to be a bulldozer,” he warned. “The homes are going to have to be razed.”

Among the steps the CDC recommends for getting rid of mould after flooding are:

Throw out items that cannot be washed and disinfected, such as mattresses, rugs and carpets, upholstered furniture and books.

Remove and discard wet or contaminated drywall and insulation.

Thoroughly clean all hard surfaces, including floors, wood and metal furniture, counters, appliances and plumbing fixtures, with hot water and laundry or dish detergent.