Matt Kelly (the "Mold Man") answers whether keeping indoor relative humidity at or below 50% will control mold. He explains there are three mold groups—hydrophilic (need lots of water), mesophilic (happy at typical indoor humidity ~30–50%), and xerophilic/zero-tolerant (can grow at very low moisture)—and notes that keeping RH in the low 40s (e.g., set dehumidifier to 45%) reduces rapid outbreaks but does not eliminate all mold growth. He emphasizes that remediation and removing moisture sources are necessary because some molds remain active even in the 40s.
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I like Patricia's question here. And it is, will keeping the relative humidity 50% or less, will this keep mold under control? And that's the number most of us that have an understanding of mold share. Like I tell people, set their dehumidifier to 45% and then it'll work to keep it under 50. It'll keep it around there. Does that keep mold under control? Tricky question. Under control. We've got three different types of molds. We've got hydrophilic, they love water, and they just grow gangbuster when there's lots of water. That's like your stacky, you know, and ketomium, when there's a lot of water for a long time, like you have a roof leak and in your attic, the water has been slowly dripping and staying there. You get those molds growing there. But if all that mold dried up, they wouldn't be as active. But then you have mesophilic molds. Those are the ones that are active, where we keep our humidity 30 to 50%. They're happy. So does it stop their growth? Not necessarily, no. They can feed off the humidity level that we're at. Even zerophilic molds are. And zero tolerant molds require very little. Like molds in the crops that get in our grains that we get all the warnings about for mycotoxins, right. They can grow it 10% moisture. Okay, so we keep it under 50% in an effort to stop like a mad dash, a mad crazy growth, an explosion of mold. But some molds can still grow fine and be healthy under 50%. So we do, we do the best we can. We can't go too dry because then people get bloody noses and skin gets dried out and hair gets brittle. We can't keep it wet. And you have structural issues when you get too dry too. You can't keep it damper because you're just feeding. It's like fertilizer for mold. Right. So we keep it in the sweet spot in the 40s. But does it stop mold? No. Does it stop some molds from booming? Yes. But does it stop all molds from being healthy? No, they're still there. And that's why remediation and inspections are about finding those sources so that they can be removed. And then it's not, you know, as much about them growing or what the humidity is. I mean, we still have to control it, but we got to get the sources out because Even in the 40s, these colonies can still be healthy. Hope that helps.
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