Washing dilemma

Washing clothes has a huge environmental impact. Treated textiles promise to reduce wash frequency but may pose bigger drawbacks, Maria Burke reports

An estimated 70% of the carbon emissions created during the life of a cotton T-shirt are due to washing and drying – more than double the amount created in its production.¹ Washing less frequently also makes clothes last longer, an important consideration when the UK alone sends around 350,000t of used clothing – worth around £140m – to landfill every year; that’s about a third of all unwanted clothing, according to the charity Clothes Aid. Other reasons to launder clothes less frequently include reducing the amount of microfibres being washed into the environment, using less energy and water, and producing fewer greenhouse gases.

The textile and fashion industry have adopted various ‘odour-free’ technologies designed to cut down on clothes-washing, partly to meet increasing consumer demand and also to respond to environmental concerns.

Sweat is odourless but when it reacts with bacteria on skin, it produces chemicals that smell: eg 3-methyl-2-hexenoic acid, 3-hydroxy-3-methylhexanoic acid and 3-methyl-3-sulfanylhexan-1-ol, along with other sulfur-containing compounds. Controlling odour in textiles usually involves applying antimicrobials to kill or inhibit the growth of bacteria, and/or incorporating odour adsorbents.

Antimicrobial agents

Common antimicrobial agents used in textiles include silver and triclosan – a phenyl ether found to prevent gingivitis when used in toothpaste. However, the US FDA has ruled that antiseptic washes and hand gels containing triclosan are not safe and effective. Silver is naturally antimicrobial. When it oxidises, it releases silver ions that are lethal to bacteria. Silver is most commonly found in sportswear, although some T-shirts, leggings and socks also incorporate it into their fabric. Triclosan weakens the cell membrane of bacteria, causing the bacteria to die. It has been used to treat acrylic fibres and can be found in items such as Biofresh socks and underwear.

But how effective are such treatments? Claims of odour control based on lab tests don’t always translate into real life, according to Rachel McQueen, a textile scientist at the University of Alberta, Canada. Her team analysed three different textiles coated in either triclosan, a zinc pyrithione derivative, or a silver chloride-titanium dioxide compound after they’d been worn. Overall, they found the textiles were not as good at preventing microorganisms from surviving in the textile as in vitro tests had suggested.

In another trial, eight men wore polyester fabrics treated with a silver chloride anti-microbial finish. Smell tests showed no perceptible differences in odour intensity between any of the treated fabrics compared with the untreated fabrics.² Bacterial populations extracted from the treated fabrics were also not significantly lower, despite there being evidence of antimicrobial activity in in vitro testing.

‘Reduction in bacteria on a textile may not eliminate or reduce odour noticeably,’ McQueen points out. ‘Essentially, for an antimicrobial to be effective against body odour transferred to a garment, it would need to have some other mechanism inherent to also eliminate the odours, such as an adsorbent.’

However, some antimicrobials may be inherent adsorbents as well.³ For example, metal oxides, such as zinc oxides, have both antimicrobial and adsorption properties.⁴

Finishes from plant extracts also appear to work this way. For example, Young Hae Lee and colleagues at Hanyang University in Korea have investigated the antimicrobial properties of cotton, silk and wool fabrics dyed with extracts from immature pine cones⁵ and myrrh.⁶ In both studies, there was evidence that the dyes have an antimicrobial effect, probably because they adsorbed odorants from the air.

Removing body odour completely is challenging and often depends on the type of fabric involved. Studies have found that while untreated polyester fabrics perform better in smell tests than non-treated polyesters, they are still smellier than untreated cotton and wool fabrics.

‘The natural properties of the textiles such as cotton and wool do impact how odorous a garment can become,’ says McQueen. ‘When sweat is transferred to cotton, the fabric takes in more of the moisture whereas polyester, for example, will repel the water but attract many of the other oily type compounds that have come from our bodies in the sweat, and many of these are odorants, eg aldehydes.’ It’s also more difficult to wash these compounds out of polyester than cotton, while different fibres can release odorous compounds at different rates,’ she says. ‘We’re finding that polyester is more of a slow release whereas wool and even nylon can release compounds more rapidly. So, this may account for some fabrics [such as wool] being more effectively aired out than others.’

Silver leaching

One issue with the effectiveness of silver-based treatments is that silver may get washed off during laundry. Paul Westerhoff and his colleagues at Arizona State University, US, have studied how the design of antimicrobial clothes affects how well they stand up to washing and their potential to leach silver into the environment.⁷ They report that silver nanoparticles and coatings do wash off commercially available garments but at negligible levels. They also found that even low concentrations of silver on clothing kept microbes at bay.

The study evaluated four silver-containing textile products, examining antimicrobial efficacy, potential silver release during simulated washing, environmental toxicity and end-of-life. Each sample used a different method to integrate silver onto polyester: covalently tethered silver nanoparticles, electrostatically attached silver nanoparticles, a silver-salt coating, and metallic-silver-coated fibres.

The team found that each method of attaching silver was effective for reducing bacterial growth. Although washing the fabrics did release silver, it did not affect their antimicrobial efficacy even for textiles that retained as little as 2μg/g silver after washing. This suggests that very little nanosilver is required to control bacterial growth in textiles, they concluded.

If you use silver-impregnated clothing, it is only better for the environment if you really do wash your clothes less often. The broader benefit is actually reducing the carbon footprint of washing and drying clothing against the carbon footprint of mining the silver. So, either use less silver or wash your clothing less frequently.
Paul Westerhoff Arizona State University, US

Tests involving multiple sequential washes in deionised water with or without detergent resulted in a range of silver releases, depending on silver loading and the method of attachment. Washing mostly released ionic silver, whereas nanoparticulate silver was primarily detected in the effluent from detergent-based washes, probably because of silver precipitation.

‘The best way [to keep antimicrobial efficacy and reduce leaching] was to attach small silver nanoparticles to the clothing,’ says Westerhoff. ‘This minimised the actual use of silver and hence the long-term leaching into wash water.’

The team also found that the resulting toxicity of the wastewater due to its silver content was negligible to zebrafish embryos - a model animal used in toxicity studies. More recent experiments have found the same, Westerhoff says. However, end-of-life experiments using simulated landfill conditions showed that silver remaining on the textile is likely to continue leaching from textiles after disposal in a landfill.

‘The conclusion of the study is that if you use silver-impregnated clothing, it is only better for the environment if you really do wash your clothes less often,’ Westerhoff comments. ‘That is what the silver affords you. The broader benefit is actually reducing the carbon footprint of washing and drying clothing against the carbon footprint of mining the silver. So either use less silver or wash your clothing less frequently.’

However, the environmental impact of silver leaching into water systems remains under debate. For example, in 2018, the Swedish Water & Wastewater Association, Svenskt Vatten, tested nine sportswear items treated with silver and found that after ten machine-washes the treated clothing items had lost on average 72% of their silver content.⁸ The report said that biocidal silver leaching from treated textiles was the largest known source of silver in Swedish treatment plants, and called on manufacturers and retailers to phase out any silver-treated textile articles, and for consumers to avoid buying ‘anti-odour’ labelled items.

However, some researchers believe that levels of leached silver are too low to cause a problem. Zhiqiang Hu, assistant professor of civil and environmental engineering at Missouri University, US, has studied whether silver nanoparticles harm wastewater treatment and sludge digestion.⁹ ‘I am not concerned about the release of silver nanoparticles into sewage because their concentrations are relatively low – in parts per billion or lower – and after wastewater treatment, there is a much lower level of nanosilver released to the environment.’

There had been concerns that silver nanoparticles could inhibit the ‘benign’ bacteria used in wastewater treatment. ‘The levels of such nanoparticles are not high enough to present a threat to water systems,’ he concludes. ‘In our previous study [in a membrane bioreactor activated sludge system], effluent water quality was not affected by the long-term nanosilver exposure at 0.1 parts per million of silver. At higher concentrations, however, silver nanoparticles may negatively affect wastewater treatment processes, such as nitrification, a microbial process by which ammonia is oxidised sequentially to nitrite and nitrate.’

Hu also points out that data on NSPW nanosilver – the active ingredient approved by the US EPA in 2015 for use as a materials preservative in textiles – indicate that the leach rate derived from NSPW-treated textiles is below the detection limit.  

Other approaches

Netherlands materials specialist Parx Materials claims its antimicrobial technology prevents bacteria from adhering to surfaces at all, rather than killing it. ‘Silver is effective in killing odour-causing bacteria, but that does not mean it is safe,’ says Michaël van der Jagt, CEO of Parx Materials. ‘As these technologies can end up anywhere; on the skin, in the air and in the wastewater from washing-machine cycles, the long-term impact should not be underestimated. Our technology, Saniconcentrate, does not kill bacteria. It does not have a biological action. We only prevent the attachment of dirt and germs on the surface. And this is an intrinsic property in the material that does not leach out or wash away. So there is nothing harmful inside and nothing comes out.’

Saniconcentrate is a biocompatible plastic additive containing trace amounts of zinc, a nutrient element that the body uses to fight off bacteria and viruses. It works by mimicking the physical characteristics of human skin, explains van der Jagt. ‘Our skin prevents attachment so that germs cannot colonise the surface. When an infection occurs on the skin, this is the place where bacteria are able to grow and cause a problem. And in many cases, this is related to a low level of zinc in the skin, such as in acute acne.’ 

Saniconcentrate is generally used in granule form and blended with the textile during the manufacturing process. The company has tested Saniconcentrate in socks made from a mix of polypropylene and PET yarn. The socks were very effective in preventing bad smell, reports van der Jagt. After as many as 50 washing cycles, the socks showed antimicrobial performances ‘far above the set minimums’.

The socks also had an unexpected outcome for patients with the skin condition beriberi, caused by thiamine deficiency. People with beriberi often have sweaty and smelly feet. In tests with 86 people, the company reports that 80 reported smoother, softer skin after wearing the socks for 15-20 days. The company suggests that zinc deficiency in these patients may mean that their skin has a ‘decreased defence mechanism’. It stresses there is no migration of zinc from the material, but merely contact with the skin delivers clinically beneficial effects.

The US manufacturer Thompson Tee takes another approach; it treats clothing with hydrogen peroxide to prevent odour. Hydrogen peroxide is used as a disinfectant because of its ability to kill bacteria by exposing them to oxygen. Thompson Tee’s Odor Shield technology uses a natural hydrogen peroxide-based solution and reportedly can eliminate 99.9% of bacteria when infused in fabric. It involves a patented ultra-thin hydrophilic membrane within a fully integrated layering and stitching system engineered to block moisture. The company claims that unlike chemical treatments, its Odor Shield will not wash off over time, and holds up for at least 70 wash cycles.

Meanwhile, Pangaia, a material science company headquartered in London and New York, uses a treatment derived from the peppermint plant (Mentha piperita) on its clothes to control odour. The company says PPRMINT is a durable broad-spectrum antimicrobial treatment, which neutralises and prevents the growth of odour-causing bacteria. The peppermint essential oil is extracted by steaming and without the need for any solvents or other chemicals. The company claims the antibacterial effect lasts as many as 50 washes, without impacting the material’s texture, colour or other physical properties. It has piloted the treatment on a few styles, starting with T-shirts, and plans to gradually add to the rest of its clothing ranges later in 2021.

Adsorbents

The most common adsorbents for textiles are activated carbon, zeolites and cyclodextrins. Activated carbon fibres have a high surface area and a large pore volume. They’re mainly used in healthcare, to control odours from wounds, and protective clothing. For example, Flexzorb, made by UK company Chemviron, is a lightweight activated cloth, first developed for the Ministry of Defence for use in chemical warfare suits. Activated carbon has also been incorporated into clothing for hunters to reduce the risk of detection by wild animals. Manufacturers claim reactivation of the carbon occurs through washing or tumble-drying.

Zeolites are microporous crystalline materials consisting of aluminosilicate components. Zeolites have been incorporated into cotton and polyester fabrics for odour control. For example, Sciessent’s Lava products use zeolite technology to attract, absorb and degrade molecules. Zeolite adsorption capacity regenerates on washing, while the company claims its Lava XL range ‘self-regenerates’ between washes.

However, McQueen notes that although there’s been some testing of activated carbon in polyester and cotton fabrics, with reports of odour reduction, little is known about the ability of the fabrics to retain performance after washing, or how activated carbon affects the feel of the fabric. She also adds there is little evidence of the odour-reducing capabilities of zeolites incorporated into clothing and other reusable textiles.

Derived from starch, cyclodextrins (CD) are natural cyclic oligosaccharides. They can trap different molecules with non-covalent bonds. In textiles, CD treatments can adsorb odorants or mask unpleasant smells by incorporating fragrances. Diego Alzate-Sanchez and colleagues at Northwestern University, US, have shown that CD treatments can work.¹⁰ They found cotton treated with a beta-cyclodextrin polymer captured organic pollutants such as styrene and aniline from contaminated air and water.

However, despite all these new approaches for keeping textiles fresher for longer, the environmental benefits of washing clothes less often will only kick in if consumers change their behaviour.

>References
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