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Research for a doctoral thesis found that thousands of chemicals (from manufacturing the textiles and clothes) remain on finished clothes, and many of these remain after washing. Bottom line: wash new clothes before wearing! From Science Daily:

Toxins remain in your clothes

Thousands of chemicals are used in clothes manufacturing. Researchers at Stockholm University have examined if there are chemicals in the clothes we buy as well. Several substances related to health risks were identified and not even organic cotton was a guarantee for non-toxic textiles. In a new thesis 60 garments from Swedish and international clothing chains have been tested. An initial analysis found thousands of chemicals in the clothes and around a hundred chemicals were preliminary identified. Several of the substances were not on the producers' lists and are suspected to be by-products, residues or chemicals added during transport.

"Exposure to these chemicals increases the risk of allergic dermatitis, but more severe health effect for humans as well as the environment could possibly be related to these chemicals. Some of them are suspected or proved carcinogens and some have aquatic toxicity," says Giovanna Luongo, PhD in Analytical Chemistry at Stockholm University.

Depending on occurrence, quantity, toxicity and how easily they may penetrate the skin, four groups of substances were chosen for further analysis. The highest concentrations of two of these, quinolines and aromatic amines, were found in polyester. Cotton contained high concentrations of benzothiazoles, even clothes made from organic cotton.

The researchers washed the clothes and then measured the levels of chemicals. Some of the substances were washed off, with a risk of ending up in aquatic environments. Others remained to a high degree in the clothes, becoming a potential source of long-term dermal exposure. It is difficult to know if the levels of these harmful substances are hazardous, and what effects chemicals in our clothes can have in the long run.

Another interesting study that makes you think about our microbiome. From NPR:

Stinky T-Shirt? Bacteria Love Polyester In A Special Way

Anyone with a drawerful of T-shirts knows that the synthetic ones can get sour after just a brief jog, while old-school cotton T-shirts remain relatively stink-free all day. And now science explains why. The bacteria that flourish on a sweaty polyester T-shirt are different from those that grow on cotton, researchers at the University of Ghent in Belgium found. Polyester makes a happy home for Micrococcus bacteria, while Staphylococcus, a common armpit denizen, was found on both poly and cotton.

Microbes love the cozy warmth of the human armpit; it's like a trip to the tropics without ever having to leave home. And it's crowded in there. Those microbes eat compounds in sweat and generate odors, which support a flourishing deodorant industry. 

The scientists asked 26 volunteers to take a spinning class while wearing shirts made of cotton, poly or blends. The shirts were then incubated for a day, and the microbes extracted and DNA fingerprinted. Volunteers also had their armpits swabbed. 

It turns out the bugs on the shirts are different from the bugs in the pits. While Corynebacterium is thought to be the main cause of armpit body odor, there was no Corynebacterium on the clothes. Instead, Staphylococcus flourished on cotton and poly, and Micrococcus, bacteria also known for making malodor, loved polyester.

He's also trying to help people with excessive body odor by giving them armpit bacteria transplants. "We have done transplants with about 15 people, and most of them have been successful," Callewaert, a Ph.D. student in applied biological sciences at the University of Ghent, tells Shots. "All have had an effect short term, but the bad odor comes back after a few months for some people."

Manufacturers have tried to make polyester fabric less hospitable to bacteria by impregnating it with antimicrobials like silver nanoparticles or triclosan. Both products have been criticized as having potentially negative impacts on the environment, and there are few data on how they might affect the wearer. Callewaert thinks the ultimate solution will be something more organic — supplant bad bugs with good ones.