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Question from Arash

Hi Debra,

I’ve used your site as a great resource over the past few months and my first go-to. I thank you for all you do.

I couldn’t find an answer to this and if you have any info maybe others would find it helpful too.

I’ve found out that the previous owners of our new house had permethrin sprayed on all the floorboards, top side and bottom, and the stairs too.

This was done over 20 years ago, but I don’t know how much of it will have soaked right into the wood and maybe (on the upper floors) into the drywall of the floor below.

Having failed to find much info on whether it offgasses and to what extent, I decided to just assume it does still off gas.

Also, apparently the dust under the floorboards will have soaked up a lot of permethrin, so dust coming through ceiling light fittings and up through floorboards is definitely a problem even if off gassing is not.

I was just wondering if you had any advice on how best to address this.

I’m thinking maybe HDPE or LDPE sheeting all over the floorboards and taped around the perimeter of each room to seal it all in. Then new flooring on top. Marmoleum or solid wood would be healthiest but they can’t be floated so would have to be nailed into the floorboards thereby breaking the plastic seal. Or glued to plywood which has formaldehyde issues.

Also unsure how to address the dust/vapor movement through the ceiling light fittings… maybe fire proof lighting as that creates a bit more of a barrier but not a complete one.

Not to mention the stairs, I don’t know how practical it would be to seal them all in with LPDE / HDPE. And the under-stairs storage which houses the gas/elec meters etc can’t really be sealed using plastic so don’t know what to do there.

Any and all advice would be very gratefully received.

Thanks Debra.

Debra’s Answer

First, Marmoleum IS available floating and so are hardood floors so that IS an option if you want it.

And second, your Permethrin exposure is not particularly a problem.

Permethrin is widely used on clothing for protection from ticks. “Exposure risk of permethrin-treated clothing to toddlers is 27 times below the EPA’s Level of Concern (LOC).” Permethrin Fact Sheet.

Not only is Permethrin not particularly toxic, you have time on your side. Here is a great article about what happens to pesticides over time: NATIONAL PESTICIDE INFORMATION CENTER: Pesticide Half-life.

About Permethrin it specifically says:

Each pesticide can have many half-lives depending on conditions in the environment. For example, permethrin breaks down at different speeds in soil, in water, on plants, and in homes.


  • In soil, the half-life of permethrin is about 40 days, ranging from 11-113 days.
  • In the water column, the half-life of permethrin is 19-27 hours. If it sticks to sediment, it can last over a year.
  • On plant surfaces, the half-life of permethrin ranges from 1-3 weeks, depending on the plant species.
  • Indoors, the half-life of permethrin can be highly variable. It is expected to be over, or well over, 20 days.

Well over 20 days. You’ve had 20 years. It’s highly doubtful there would be any Permethrin left in your home at this time.

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