Why bamboo socks do not smell

Bamboo socks do smell less than cotton or polyester ones. The reason is not the one printed on the packaging. Bamboo viscose pulls moisture off your skin and holds it inside the fibre instead of leaving it on your foot, and the bacteria that make feet smell need that moisture to work. There is no antibacterial bamboo compound left in the yarn — the FTC has fined major retailers for saying there is.

Your feet do not smell. The bacteria on them do.

Fresh sweat is close to odourless: water, salt, urea, a few amino acids. The smell is a by-product. Bacteria already living on your skin feed on what sweat and skin lipids give them, and what they excrete is what you notice. The main offender in foot odour is isovaleric acid, the sharp cheesy note — a 1990 study in the British Journal of Dermatology found it in every subject with foot odour and in none of the subjects without, and reproduced the smell in the lab by incubating sweat and lipid together.

Which means a sock has exactly one lever to pull. It cannot kill the bacteria in any way that survives a wash. It can take away the warm, wet film they need. Drier foot, slower bacteria, less acid, less smell. Everything else is marketing.

What bamboo viscose actually does

Bamboo viscose is rayon. Bamboo cellulose is dissolved in chemicals and spun into a new fibre, and none of the plant's own chemistry comes through the other side. What does come through is the behaviour of regenerated cellulose, which is unusually thirsty.

Moisture regain is the share of its own dry weight a fibre carries as water under standard conditions. The commercial figures used in textile engineering courses are:

Polyester barely absorbs anything, so sweat sits on your skin and in the air gaps. Cotton absorbs well and then holds on — a cotton sock that is damp at 2pm is still damp at 6pm, pressed against your foot. Viscose takes up more than cotton does and gives it back to the air faster, so the fibre does the holding instead of your skin.

Fibre choice measurably changes which bacteria grow, too. In a study of 26 people's shirts after an hour of indoor cycling, published in Applied and Environmental Microbiology, a trained panel rated the polyester shirts "significantly less pleasant and more intense" than the cotton ones, and micrococci grew almost exclusively on the synthetics. Same people, same sweat, different fibre, different smell.

The antibacterial claim, and the $1.3 million

Nearly every bamboo sock listing on the internet says the fibre is "naturally antibacterial", usually crediting a compound called bamboo kun. Set aside whether raw bamboo has it. The question is whether it is in the sock, and the FTC's position on rayon is blunt: "there's no trace of the original plant in the finished rayon product."

This is not a theoretical dispute. In December 2015 the FTC collected $1.3 million in civil penalties from Nordstrom, Bed Bath & Beyond, Backcountry.com and J.C. Penney for labelling and advertising rayon textiles as "bamboo". In the same round of actions the Commission charged that J.C. Penney had "falsely claimed bamboo gave the products anti-microbial properties" and called Backcountry's anti-microbial claims misleading. The guidance says a seller wanting to claim the fibre keeps antimicrobial properties from the plant needs its own scientific evidence, because "relying on other people's claims isn't substantiation."

We sell bamboo viscose socks. We are not going to tell you the fibre kills anything.

Four things decide this, and fibre is only one

Your shoes hold more moisture than your socks. A closed shoe worn two days running never dries out, and a fresh sock goes into a wet environment. Alternating two pairs of shoes does more for smell than changing fibre.

Thickness beats material. A heavy cushioned crew sock inside a warm boot will out-sweat a thin quarter sock whatever either is made of. Match the sock weight to the shoe, not to the season.

Fit. A sock that slides and bunches makes damp folds. That is a local wet spot, which is where smell starts.

Washing residue. Fabric softener leaves a coating that stops cellulose absorbing — it defeats the one property you bought the socks for. Skip it. Wash cool or warm, turn them inside out, and keep them out of a hot dryer; viscose is weaker wet than dry and a hot tumble shortens its life. If a pair smells after washing, that is trapped residue rather than fibre failure, and a longer soak usually clears it.

What a sock will not do

No sock treats a medical condition. Odour that does not change when the socks are clean and the shoes are dry, broken or peeling skin, persistent itching, discoloured nails — those are questions for a clinician, not for a fibre.

What bamboo viscose socks are is comfortable, smooth against skin, and better than cotton or polyester at moving moisture away from your foot over a long day. Less moisture is less smell. That is the mechanism, it is unglamorous, and it is the whole claim.

Shop the Clothing collection, or go straight to the cushioned crew socks and the thin quarter ankle socks.