Matt Kelly compares petri dish mold tests to lab-based ERMI and EMMA tests. Petri dishes are cheap and can spot heavy localized contamination, but results depend on airflow and placement, making them unreliable for measuring overall household mold. For accurate baseline measurements, especially with health concerns, use DNA-based dust testing instead.
Let's talk a minute about mold testing petri dishes. A lot of times people get scared off at the price of an Ermie. I would too. I mean, nobody wants to spend money on this kind of stuff just to be given bad news, mind you. So say you want to find out if your house has mold. You pay 2, 3, $400 for an hermi and testing and then you find out you have mold and now you've got a giant bill to try to find and fix it after. It's all bad news. I get it. But the temptation sometimes is to grab the cheapest option at your local hardware store. Home Depot, do the petri dish test. The problem is the petri dishes are not real reliable. Okay. An ERMI test, Environmental Relative moldiness index, uses DNA analysis of the dust to look for the DNA of spore and spore fragments within that. And you can do the add on for mycotoxins as well. But with a petri dish, all that's being measured is if a spore happened with the current airflow of your house when you had the petri dish out for an hour, happened to land on that, and that's the material that it likes to grow on and it grew in time to be measured. And then they'll tell you what species that is. Now the problem is what if the spores, or worse spores don't land in that? What if enough spores don't land in that over that hour most people go around, set them in and there's no air movement. What if your H Vac isn't running? What if there's nobody walking around to create any air movement and not many spores are moving and dropping? Okay, so there's a lot of variables that create a problem for petri dishes. So am I going to put all my faith and trust in petri dish what the petri dishes say no. Can they be helpful and a cheap first step? Yeah. If you don't have the budget to do it right, then go for the petri dishes. And if all of a sudden it's a blowout, like, yeah, there's like 25 colonies growing on here, then you know, you lucked out, it's for very cheap, you found out there's a big problem. But if you do it and you don't have a lot of colonies growing, does that mean you don't have a lot a mold problem in your home? No. So it kind of only works one way. You can get lucky with it. But, but another, I mean another way that it is good though is if you do have a real bad mold problem and you're not sure where, if you put them in like every single room and one or two are clearly worse, even if they're not like this one has 25. But if all your rooms are like 2 or 3 and a couple rooms are like 6 or 7 or 7 and 8 or something like that. Right. Is not extremely bad, but it tells you the mold problem itself might be in these areas. So that's beneficial of a petri dish. Where I do like, petri dishes are for tap testing. So if you go to your couch and you want to see, because couches, you know, they're absorbing all those spores and you go to your couch and tap it on the couch face down, right. So that the spores puff up. And then you do that and see what grows. You go, oh, my goodness. You can see that the couch is holding a lot of spores, a lot of mold. You know, you can even do it on the pet. You can do it on the carpet. Carpets are good, especially if they're in the basement. You can do a tap test on the carpet and see. So I like that aspect about petri dishes. But overall, if someone's looking to find out if their house has a mold problem, if it's mold toxic, if the mold loads too high on it, I would never use a petri dish and trust it for that. But like I said, if you do do it and it ends up high, bingo, you got lucky. You know, there's a problem. I would still say, I would still want to see an ermie to see which molds they are, you know, and the concentrations, like, do I have, you know, a hundred times high stacky as what should be there. You know, I'd want to know that because I'm an analytical person and I like to do AB testing. I want to know that what I'm doing is working. So I need to be able to retest. I can't retest accurately if I don't have a baseline. So petri dishes, they have their spots that I like them. But for overall mold toxic load on a home, I don't like them for that. But hey, they're cheap. If you want to do it, try it, go ahead. I just would never put my faith in that as the be all and end all. When testing a home, especially if you got sick kids or your wife's sick or something like that, you really need to figure out what's going on. You're going to have to spend more than 30 bucks. You're going to want to do the full Ermi or Emma would be good to do. All right, that's my thoughts on petri dishes.
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