Actual AP headline yesterday: "Banana Boat recalls sunscreen due to fire risk." CNN, ever one for a scare, upped the ante with their headline: "Sunscreen could burst into flames on skin." (emphasis mine). Did I miss the part when sunscreen formulators started including white phosphorus in the mix?!?
Kidding aside, I'm not talking about "toxic chemicals" here; sunscreen already occupies a special place at Just Like Cooking due to the clouds of chemophobia swirling around it. The fire risk appears to involve two factors: engineering and consumer usage. An Energizer Holdings* corporate spokesman said the canister spray valve applies too much product, which leads to longer-than-average drying times. The exacerbating factor? Customers suffering burns engaged in barbecuing and welding immediately after application.
Well, what is so flammable about spray-on sunscreen, anyway? The ingredients list shows all the usual suspects: UV blockers, vitamins, surfactants, and polymers - none of which are particularly flammable. Two components catch my eye, though: isobutane and SD Alcohol 40. One commonly finds isobutane, the simplest branched hydrocarbon, in refrigeration coils and aerosol products. It could certainly ignite, but high volatility means that most of it flies away mere seconds after you spray it on. SD Alcohol 40**, however, is almost straight ethanol, and makes up a large portion of the overall spray. It would also fit the bill for physical properties: boils at 173 deg F, vapor pressure 40x lower than isobutane. A fog of ethanol slowly evaporating from your arms certainly invites ignition.
*Fast fact: Did you know Energizer Holdings owned Banana Boat? I didn't. They also apparently make shaving creams and tampons. Huh.
**The Code of Federal Regulations (CFR) actually tightly regulates specially denatured (SD) alcohols for use in lotions, oils, creams, and sprays. Here's the page for blend "40." I was rather surprised to find out that sunscreen actually contains trace amounts of complex alkaloids: quassin, brucine. The bitter taste and side effects discourage drinking SD-40 for recreation.