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Why do you need to reapply sunscreen?

Asked by Jake 1.26K Rep.

Hi Dr. Schultz,

Why do we have to reapply sunscreen every 3-4 hours even if not doing any physical activity or not being out in the sun?

That would mean applying sunscreen to your face 3 times a day. Who can do that? I don’t know anybody who does that.

Thank you.

Jake edited question
    Garrett Smith

    this is an response from Garrett

    Garrett Smith answered
      Dr. Schultz 1.26K Rep.

      Everyone perspires all the time on most places on their skin, even when not doing physical activities. It’s called “insensible perspiration” because you don’t feel or see it but it occurs as part of your body’s way of regulating your temperature and metabolism.

      The reason you don’t feel it is because it is so light that it evaporates before it forms the water droplets that you feel when you perspire from exercise. This continuous ongoing “insensible perspiration” is constantly washing off your sunscreen, hence the need for reapplication even if “not doing any physical activity.”

      If you are indoors in a room without windows, then you need not reapply your sunscreen until you go outside. If the room is windowed, since window glass allows all the UVA rays (which cause skin cancer and premature aging of the skin) to pass through, then you still need to reapply sunscreen approximately every 4 hours on parts of your skin not covered by clothing.

      Dr. Schultz answered

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