I used up my bottle of clothing repellent treatment and started searching for something cheaper including repurposing lawn or animal treatments. Most all of them use pyrethrin (natural) or permethrin (synthetic) insecticides. The natural form can trigger allergies while the synthetic is more common but possibly less healthy depending on what studies you look at. They're often combined with synergists like piperonyl butoxide (PBO) which reduce insect resistance to them.
There are concerns over toxicity to humans but both (and PBO) are found in medical treatments for lice and scabies, typically with concentrations in the 0.5-4% range. Animal treatments (for most any mammal except cats whose livers can't neutralize it) are available in concentrations in up to 10%. Lawn and garden spray concentrations vary widely. Clothing treatments are typically 0.5% permethrin, which is probably a regulatory limit, and don't contain PBO (Ben's, Coleman, Repel, Sawyer).
The main problem isn't these chemicals - it's the unknown "inert" ingredients. Most everything I've found uses "petroleum distillates" which is a non-specific category of chemicals. On safety data sheets their names and/or quantities are usually redacted as trade secrets. According to an article by Consumers Reports some clothing treatments have additives to enhance adhesiveness but I suspect that's only what the manufacturers told them without providing specific chemical data. Most insecticides are formulated to have long surface adhesion duration regardless of application target.
While trying to find and compare water-based and petroleum-based mixes I discovered an excerpt from a 2013 research book titled Biotextiles as Medical Implants by D. Tessier. In the search summary the chapter Surface modification of biotextiles for medical applications discusses durability of permethrin treatment. According to Tessier water-based treatments last for two weeks or two washings, oil-based for six weeks or six washings. Treatments last much longer if kept in sealed light-proof bags. I haven't found any specific info on the durability of pyrethrin but permethrin is considered to be longer-lasting due to better water and sunlight resistance.
According to a fact sheet (PDF) from the National Pesticide Information Center at Oregon State University, PBO breaks down in 3-8 hours in sunlight which may explain why it's not found in clothing-specific treatments.
Factory-treated clothing reportedly has much longer-lasting effectiveness than spray treatments. My guess is they are using higher concentrations and perhaps enhancing the treatment bonding using elevated temperatures and/or pressures.
It seems like the best treatment would be water-based permethrin solutions but I haven't found any without PBO which only provides a short-term benefit. The closest to ideal is MGK Sector Misting Concentrate which is 10% permethrin, 10% PBO, and less than 10% petroleum distillates. The permethrin/PBO ratio of 1:1 is unusual since most mixes are 1:5 or 1:10 but would still have to be diluted. The presence of unknown petroleum distillates is annoying in a "water-based" product but it is much less than regular sprays. I could avoid it with other solutions but would have to accept a much larger amount of PBO. The only other option would be to buy concentrated permethrin in bulk off Alibaba and mix it myself.
Not surprisingly it comes down to cost, labor, and safety. I'll have to study this a bit more.