A friend asked me this question the other day and my automatic answer was, “whenever they need it?” Of course, after thinking about it some more I decided more research was required.
The most cited article about washing your jeans comes from the University of Alberta, Canada. It’s an informal project and the results appear to be that there is no difference in bacterial content for a pair washed after two weeks of wear and a pair washed after 15 months of wear.
However, the jeans were “raw” denim. Raw denim is a recently popular style and means that the fabric isn’t washed after being dyed and set. Proponents of raw denim suggest brushing (how 1800’s) or freezing to keep them clean and odor free, however, a scientist says that’s not going to help. This makes sense to me as there are lots of bacterial species that can easily survive freezing, warm back up, and keep going.
So… I have a lot of questions about this informal project…
Was there underwear involved? Was it worn 100% of the time? 50%? What about cotton poly blends? Would bacteria live longer on synthetic blend jeans? What kind of food was on the jeans? How many hours were they worn? How many consecutive days? What kind of detergent was used for test washes? Was the outside tested? Was the bacterial load measured (sounds like it was taken from the crotch) higher in certain locations? Did it radiate out from a ground zero? Was there bacteria on the back pockets? What about the cuffs? Did the hem touch the ground? Would there be more bacteria in tighter jeans vs. a looser pair? Where did the jeans travel? Private or public transportation? Large events? Was there bias in how the denim was treated (raw denim jeans can cost significantly more and I’d be less likely to wipe burrito drip on an expensive pair of pants).
And most importantly… What did they smell like after a year?
I had a difficult time finding scientific papers addressing this topic but jumping in was very educational!
The majority of research I found comes from forensic journals, which makes sense (apparently blood dilution, denim and luminol activation is a hotly debated subject!). Here are a few of my favorites from various disciplines:
Materials Science: Jeans can be made to be antibacterial by infusing the denim with silver nano particles. Infused denim prevented prevented Staphylococcus aureus and E. coli growth. Very cool! I wonder if they’ve tried copper yet? I wonder how much silver jeans would cost? Would they be stiff?
Forensics & Ballistics: Wearing jeans when you’re shot increases your chance of fracture. Reading the materials section alone was fun, what an awesome shopping list: deer femurs, ballistic gelatin, Levis. Also has interesting illustrative femur photos!
Emergency Medicine: Jeans are a good idea if you’re hiking in areas where there are venomous snakes. Denim is a fantastic material for stopping “venom expenditure” from rattlesnakes!
The cotton used to make denim doesn’t appear to have any bacteriostatic (bacteria can adhere but not grow) or antibacterial (actively fights bacterial adhesion & growth) properties. There has been lots of research on this topic and lots of research about creating antibacterial fabric.
Sadly, denim doesn't have any magical bacteria fighting properties so it's still gonna get stinked up if you go commando or use your jeans as a napkin. But it does sound like the jeans of the future will be kind of magical!
So I think my initial answer stands, depending on what you put your jeans through, wash them as often as they need it. Clearly more research required!