These are non-toxic mattress covers to seal in flame retardants, dust mites, pesticides, scents, or mold spores. These options are vetted by the chemically sensitive and mold sensitive.
Anything with a pore size of zero (plastic or aluminum) should fully block flame retardants, mold spores, and mycotoxins.
As for blocking gasses, foil is better than plastic, as plastic will eventually absorb the VOCs/scents and so they will come through to the other side. However, plastic can still be used in the short term, and disposable plastic options can be swapped out as needed.
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1. Polyurethane-Lined Covers
If you have an old mattress and you want to block flame retardants from migrating out of the mattress I would use a cover like Organic Lifestyle’s Bed Bug Cover. This has a pore size of zero so it should block flame retardants (though the company has not tested it for this purpose).
It is also waterproof and will protect you from dust mites and bedbugs as well.
Another good use for this cover is when you are moving a mattress into a high-humidity environment like a trailer, or you have to put a mattress on a flat surface like the floor for a while.
The polyurethane did have an odor to me but after some offgassing, I found it to be great. Other sensitive folks have agreed with this.
2. Polyethylene Covers
Polyethylene covers are the least toxic plastic cover but they do not hold up very well after washing and are not usually marketed as mattress encasements.
I have used the U-Haul mattress encasements for temporary protection from contamination or if I need to put a mattress on the floor in a hurry. These are disposable (not washable), though they do last quite a while. They are not usually contaminated by scents if you buy them through U-Haul.
3. Polypropylene Covers
There are polypropylene covers as well, they hold up a little longer than plain polyethylene.
This All-in-one cover is a mix of polyethylene and polypropylene and is reported to be very tolerable by someone very sensitive to plastic.
Plastic covers do slow down offgassing somewhat, from the foam or secondary scents, but usually, these VOCs eventually soak into the plastic layer, so it’s not a permanent solution.
4. Aluminized Tarps
If you do not have a choice but to use a mattress that is offgassing or fragranced and you want to block the odors/chemicals completely, you can use aluminized tarps and tape.
I have used these to sequester many beds, especially when traveling.
Discard as soon as the aluminum flakes off, or face the aluminum down. These are not totally free of a plastic odor and plastic offgassing so you would need to check them out if you are sensitive.
The other option would be thick mylar bags (not the thin sheets).
5. Cotton Dust Mite Covers
That brand is not organic but they were good for me after one wash. I’m really happy with them as they have reduced my allergies and the price is a steal.
For organic cotton versions, I like these pillow covers. These are still reasonably priced.
Barrier cloth can also work here.