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Sunscreen plus swimming pool chemical may be a dangerous mix
A number of sunscreens contain titanium dioxide nanoparticles coated with aluminum hydroxide, which are considered one of the safer options for sun protection.
Safer, that is, until you expose them to pool water containing chlorine.
The particles are coated to prevent the titanium dioxide from coming in contact with water and forming harmful chemicals known as oxygen-containing free radicals, a.k.a. reactive oxygen species, which can damage lipids, proteins, and DNA.
In this study the coated particles were unchanged by plain water but the coating was degraded when the chlorine levels were greater than 0.4 ppm (most chlorinated swimming pools contain 1 – 3 ppm chlorine), allowing the exposed titanium dioxide to produce free radicals.
Not all titanium dioxide nanoparticles are coated with aluminum hydroxide; polymers, silicon, magnesium, and zirconium hydroxides are also used Additional research is needed to determine those other coatings are affected by chlorinated water or not.
In the meantime you might want to rely on high SPF clothing for protection before swimming in chlorinated pools and save the sunscreen for afterwards.