Sunscreen indoors: who actually needs it

Window glass blocks most UVB but far less UVA — the wavelength most associated with pigmentation and photoageing.

If you sit beside a window for several hours a day, or drive a great deal, reapplication matters more than the SPF number on the bottle.

• A broad-spectrum product, applied properly, beats a higher SPF applied thinly
• Reapply every three to four hours of continuous exposure
• Screens themselves emit negligible UV; the window next to them does not

If you are being treated for melasma, this is not optional.