In heavily polluted areas, we all know that we should try to avoid inhaling whatever toxins are floating around as much as possible. That’s why when pollution gets especially bad in China, people are advised to stay indoors, and it's one of the reasons behind the rise in surgical masks being worn in Tokyo, Japan.

But a new study has found that when it comes to certain airborne pollutants, called phthalates, our skin can absorb just as much as we’re breathing in. "We're big sponges for these chemicals," lead researcher, John Kissel from the University of Washington in the US, told Science News.

Phthalates are a group of 'semi-volatile' chemicals that are used to make plastic soft and flexible, or as dissolving agents for other types of materials, and are found in all kinds of cosmetics, fragrances, and household cleaners. Derived from oil, around 2 million tonnes of phthalates are produced across the world each year, and more than 20 different types are in common use.

Over the past 50 years, they’ve become the most widely used 'plasticisers' in the world, but growing concern over what prolonged exposure - through indoor air, dust, and food packaging, for example - is doing to our health has seen some types banned in Europe and the US.

Studies have shown that phthalates can end up in our bloodstream, breast milk, and urine, and they’ve been classified as 'endocrine disruptors', because of the way they affect our body's hormonal systems, such as the oestrogen and androgen hormone systems.

Preliminary research has linked certain phthalates to the incidences of asthma, with a 2008 study suggesting that heated PVC fumes could contribute to development of asthma in adults, while phthalate exposure in the home could put children at a higher risk of asthma and allergies. There are also indications that exposure could lead to a higher risk of breast cancer, but the research has not yet been definitive.

Kissel and his colleagues wanted to investigate the effect of 'dermal uptake' - or absorption through the skin - on the levels of phthalates found in human bloodstreams. They recruited six healthy male participants, and exposed them to elevated air concentrations of two types of phthalates: diethyl phthalate (DEP) and di(n-butyl) phthalate (DnBP). According to Tim Sandle at The Latest News, DEP is used as a solvent in personal care products, such as moisturiser, and DnBP is used as a plasticiser in products like nail polish.

The six volunteers were exposed to the chemicals over a 6-hour period in a special chamber, first with specialised breathing hoods on that prevented them from inhaling any of the phthalates, and then without hoods the following week. The only other clothes they were allowed to wear during exposure was shorts, and they were placed on a restricted diet and restricted use of personal care products 12 hours before entering the chamber and until their urine was collected 66 hours later.

"Metabolite concentrations were lower when the participants were exposed to chamber air while wearing a hood, but the levels were still substantially higher than levels measured before the participants entered the chamber, indicating significant uptake of DEP and DnBP while participants were wearing a hood," the researchers report in the journal Environmental Health Perspective.

The team found that the dermal uptake of DEP was about 10 percent higher than its inhalation intake, and the dermal uptake of DnBP was 82 percent of its inhalation intake. And the older the participant, the higher the dermal uptakes of both DEP and DnBP from the air. They said that based on the very limited sample of six, the impact of age was surprisingly strong. "The uptake of DEP by the 66-year-old is five times greater than that of the 27-year-old, while the uptake of DnBP is seven times greater," they report.

The researchers are yet to test how wearing more than just shorts would affect phthalate levels, and yep, that sample size is tiny, but the results warrant some follow-up investigations. Skin is the largest organ in the human body, so if it’s really not protecting us against these commonly used chemicals, that’s a problem. Earlier research has shown that semi-volatile chemicals like phthalates tend to pass through the skin relatively slowly, but they could still build up to damaging levels over several years.

"[I]f the whole body is exposed, then even low rates of exposure can deliver what turns out to be nontrivial amounts of these chemicals," Kissel told Janet Raloff at Science News.