Unilever has pulled more than a dozen aerosol dry shampoos because, the company said, they may contain “elevated levels” of benzene, a naturally occurring chemical that can be a carcinogen at high levels of prolonged exposure.

The recall, announced last week by the consumer-goods giant, is the most recent related to the rates of benzene contamination in various aerosol products, including some sunscreens and deodorants.

The Washington Post spoke with experts in aerosols and cosmetics about the recall and the health risks associated with continued exposure to benzene. Here’s what they said.

Which dry shampoos are being recalled by Unilever?

Unilever has issued a voluntary recall for aerosol dry shampoos produced before October 2021 under the brands Bed Head, Dove, Nexxus, Suave, Rockaholic and TRESemme in the United States. The company said in a statement that it hasn’t learned of any “adverse events” relating to the products in the recall and that an “independent health hazard evaluation” found that daily exposure to benzene in the recalled products is not expected to cause health problems.

“Unilever U.S. is recalling these products out of an abundance of caution,” the statement said. “Consumers should stop using the affected aerosol dry shampoo products.”

The company is offering refunds for the specific products, which can be found here.

What is benzene?

Benzene is a colorless or light yellow liquid that smells sweet and is highly flammable, according to the Centers for Disease Control and Prevention. The agency says it’s one of the top 20 chemicals used in the United States. It is a “building block” for other chemicals and materials, according to the American Chemistry Council.

Benzene is commonly found in crude oil, according to the CDC. Companies use benzene to make plastics, resins, nylon and synthetic fibers, as well as some lubricants, rubbers, dyes, detergents, drugs and pesticides.

Experts say that we’re exposed to benzene every day in the air we breathe, especially when we fill our vehicles at the gas station. Benzene also is in certain cigarettes, detergents, glues and paints.

How does benzene end up in your dry shampoo?

Unilever said that the propellant in the spray cans of the dry shampoo was the source of the benzene and that it is working with suppliers to address the issue.

Chris Cappa, a professor of environmental engineering at the University of California at Davis, said butane, a petroleum product, is a common propellant in spray cans. If the refinement process of butane “isn’t very good,” then you can end up with gas that contains other components from the crude oil, such as benzene. “That gas is where, most likely, this benzene is coming from,” Cappa said.

“If you want to limit the potential exposure to things like benzene from contaminated spray cans, then you can make different choices about the products that you use,” he said.

Cappa said he’s less concerned about using a spray can of sunscreen outside vs. an aerosolized dry shampoo inside because the benzene will dissolve into the wider atmosphere and limit the risk of a high level of benzene exposure.

Is benzene harmful to humans?

Breathing in, digesting or otherwise absorbing benzene over long periods of time can lead to serious health issues, including cancers such as leukemia and other blood disorders, according to the Food and Drug Administration.

Benzene can slow down the amount of red blood cells produced by the bone marrow, leading to anemia, the CDC says. It can also damage the body’s immune system by changing the blood levels of antibodies. People who breathe in high levels of benzene can become drowsy, dizzy and confused, and have headaches, an irregular heartbeat and tremors.

High levels of benzene can lead to vomiting, dizziness, sleepiness and convulsions. Direct exposure to benzene to the eyes, skin or lungs can damage the tissue and lead to irritation. Some women exposed to high levels of benzene had irregular menstrual periods and a decreased size of their ovaries. “It is not known whether benzene exposure affects the developing fetus in pregnant women or fertility in men,” the CDC says.

Kelly Dobos, a cosmetic chemist and adjunct professor at the University of Cincinnati, said that benzene is “certainly dangerous” but we are exposed to the chemical every day and that the levels of contamination in these cosmetic products tend to be in the tens of parts per million. “It’s a trace contaminant,” Dobos said. “The cosmetic companies have toxicologists on staff. They do extensive research to ensure their products are safe.”

If you’re going to use an aerosol product, Dobos said, do so in a well-ventilated area, with a window open.

What other products contain benzene?

Aerosolized versions of conditioners, deodorants, antifungal deodorants and sunscreens have all been recalled in the past two years over possible benzene contamination.

Procter & Gamble last year recalled more than 30 aerosolized hair-care products, including dry shampoos and conditioners, warning that they contain high levels of trace benzene. The company issued a similar recall of more than a dozen Old Spice and Secret-branded aerosol deodorants, as well.

Homer Swei, a senior vice president at the Environmental Working Group, a research and advocacy organization, said the supply chain of the propellants, the butane or propane fuel for the spray cans, must be affected to have these high levels of benzene in each of the aerosol products.

“You are seeing more and more of these companies start to assess and investigate. They’re probably going to see more of these things,” Swei said. “I don’t think this is the end.”

Benzene is carcinogenic, he said, but the duration or level of exposure required to cause these health issues is not known. Benzene comes from several sources so it’s hard to “account for all these different types of exposure,” Swei said. People need to “avoid using these aerosols until the industry can fix these problems in the supply chain,” he said.