The news that benzene has been found at high levels in numerous dry shampoo brands has shed further light on the presence of this cancer-causing substance in consumer products.
As media have reported, 70 per cent of dry shampoos tested by Valisure, an independent laboratory in the US, found many containing high levels of the carcinogen.
The findings, released at the beginning of this month, are of particular concern because dry shampoos are administered using aerosols, so benzene may linger in the air and be breathed in over time.
Just last month, other dry shampoos were recalled because of concerns about benzene levels. Last year, Valisure raised the alarm about levels of benzene in hand sanitisers, body sprays and sunscreens, several of which were also recalled.
Here we look at the laboratory's most recent findings and consider what they mean.
What was found in the dry shampoos?
Valisure analysed 148 batches from 34 dry shampoo brands. It found that 70 per cent (103 batches out of 148) had levels of benzene above the safe limit for drugs of two parts per million (ppm) specified by the US Food and Drug Administration.
Some drugs require benzene for there to be a therapeutic effect, but the presence of benzene in dry shampoos, even at low levels, is not necessary.
As a result, in a "citizen petition" to the FDA, Valisure’s chief executive, David Light, and the organisation’s head of global analytics, Dr Qian Wu, said "any significant detection of benzene could be deemed unacceptable".
One dry shampoo brand contained levels up to 170 times the FDA limit, while 11 samples had concentrations more than 10 times the limit.
"These findings are the highest and most broadly detected levels of benzene observed by Valisure to-date in a consumer product category," the laboratory said.
Because the products are used indoors, where the benzene "may linger and be inhaled for prolonged periods of time", Mr Light said the detection of high levels was "a significant cause for concern".
Just last month, the FDA reported that high levels of benzene had caused Unilever in the US to issue "a voluntary product recall" for some batches of Dove, Nexxus, Suave, TIGI (Rockaholic and Bed Head), and TRESemmé dry shampoos.
The carcinogen was present because of the propellant used, and Unilever was said to be working with the supplier to deal with the issue.
What is benzene?
Benzene is a hydrocarbon that is usually obtained from crude oil. Although a carcinogen, it has valuable functions, being used today in the production of everything from insecticides to plastics.
The harm that benzene can cause was identified more than 80 years ago. It may cause short-term irritation and, more seriously, acute myeloid leukaemia and other types of cancer.
In the past, before it was found to cause cancer, benzene was an ingredient in cosmetics and cleaning products, and was even used in the process of decaffeinating coffee, Public Health England states in an online briefing document.
"Benzene is ubiquitous in the environment," the FDA said in a statement last month. "Humans around the world have daily exposures to it indoors and outdoors from multiple sources."
Some people have higher exposure because of their jobs, because of where they live (as a result of pollution) or if they smoke.
Benzene is found in petrol and vehicle exhausts, but levels are lower than they used to be.
Employees can be exposed to benzene at chemical and petrochemical plants, and oil refineries, although regulations should reduce risks.
People can take in benzene through their skin or eyes, by breathing it in or by eating or drinking substances that contain the chemical.
What is happening now over benzene in dry shampoos?
In statements reported by US media, several producers of dry shampoos highlighted in the latest results as having high levels of benzene, including Batiste and Not Your Mother’s, said that propellants from suppliers were the source of the chemical. These propellants reportedly were not supposed to contain benzene.
Some manufacturers said their own testing had not detected benzene and indicated that they would look further into Valisure’s findings.
In its citizen petition, Valisure requested that the FDA ask for a recall of high-benzene batches of dry shampoo because they were "contaminated with a known human carcinogen".
Valisure also recommended that the FDA update its regulations and clarify that there is "no acceptable level of benzene in cosmetic products and establish a reasonable detection limit".
The FDA has yet to respond publicly to the latest findings from Valisure.