Can you get a sunburn in the car with the windows up?

At a recent summer party, someone asked if you can get a suntan or sunburn in a car with the windows up. We were evenly divided on the issue. What's the answer?

D.M.,
Sutton

In a way, you're all right.

Most glass, in cars or houses, blocks the kind of sunlight, called ultraviolet B, or UVBthat causes sunburns, said Dr. Robert Stern, chief of dermatology at Beth Israel Deaconess Medical Center. So you're unlikely to get truly burned through the window. But glass does let in ultraviolet A, or UVA, light, which can cause suntans, though very slowly.

And both UVA and UVB can cause skin cancer and skin aging, so you can be damaging your skin even through sunlight filtered through a window if you have long exposures, cautioned Dr. Thomas Ruenger, a dermatologist at Boston University School of Medicine. Exact thresholds for risky exposures have not been defined, but the longer the exposure (think truck drivers), the greater the risk.

For young children, especially those in rear-facing car seats, it makes sense to lather up with sunscreen before long car trips because children get sun from both side and rear windows, noted Dr. Sandy Tsao, a dermatologist and clinical director of laser surgery at Massachusetts General Hospital. Light-shielding curtains that stick on to windows with suction cups also can help. Tinted windows keep out some light, but federal regulations prohibit tinting the entire windshield for visibility reasons, so you can't count on tinted windows as total protection, Tsao said.

Adults or children taking certain antibiotics such as doxycycline (Vibramycin) or sulfamethoxazole (Bactrim) that increase sensitivity to sunlight should also be careful on long, sunny car trips.

JUDY FOREMAN