How Much Sunscreen to Put On

Episode #196 / May 24, 2010
Did you know that the SPF of a sunscreen is decreased to it's square root if not enough is put on? And do you know how much sunscreen you actually need to apply? Dr. Schultz explains.
Ally on May 30, 2010 at 1:10am

Can ordinary mirrors reflect UVA/UVA light? My apartment faces south so I get the sun coming in pretty much all day and I can't really avoid it and I spend a few hours a day using the computer in the sun. I put on sunscreen everyday but it's a hassel to wash and reapply every 2 hours. Would sitting behind a large ordinary mirror facing the window (with the mirror part facing the window where the sun comes in and me sitting behind it) reflect UVA light?

Neal Schultz, M.D. on May 31, 2010 at 11:22pm

@Ally: It depends on whether your mirrors are silver backed or aluminum backed. Silver is a poor reflector of UV A. Aluminum is much better at reflecting UV A; it can reflect up to 90%. But if you are indoors and just doing desk work and not actively sweating, you can probably wait 3 to 4 hours to reapply sunscreen. This is a great question and will become a DermTV episode. Thanks!

alania on June 9, 2010 at 9:57pm

what about physical sunscreen?how much to put on face and neck and how often to repply?

Ally on June 13, 2010 at 4:37pm

Cool :) I can't wait!

Neal Schultz, M.D. on June 14, 2010 at 11:29pm

@Alania: The reapplication guidelines for both physical (chem free) and traditional carbon based sunscreens are the same: reapply after swimming, sweating, or at the very least, every three to four hours if not doing any physical activities. However, as will be demonstrated in an upcoming DermTV episode, you need less physical sunscreen than traditional sunscreens; use about 1/3 to 1/2 as much because the physical sunscreens are made from very small particles (called "micronized", which means the particles are made in a very small size) which spread better, give you more coverage and form a thinner layer so you don't need as much. Stay tuned for the episode which will also include some other physical sunscreen tips!

John on June 16, 2010 at 3:28pm

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 appyling sunscreen to your face 3 times a day. Who can do that? I don't know anybody who does that.

Neal Schultz, M.D. on June 24, 2010 at 9:09pm

@John: 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 slight 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 reaplication 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 UV A 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.

Katherine on June 28, 2010 at 12:05pm

What about spray sunscreens? How much do you have to spray to get the full SPF on the bottle?

Neal Schultz, M.D. on June 29, 2010 at 2:17pm

@Katherine: Good question! The problem with spray sunscreens is that it's hard to be sure you've applied it in such a way to give you even and complete coverage. Having said that, since different spray nozzles dispense a different flow of spray, and you don't know exactly how much you have used, the only general guidelines for applying a spray sunscreen I can offer at this time is to see how large of an area the spray covers (from a distance of 1 to 1 1/2 feet from the body) and then use best judgment to ensure that the spray covers all areas of sun-exposed skin evenly and completely.

Moka on July 16, 2010 at 8:28am

Is being under direct sunlight for 5-10 minutes a couple of times a day without sunscreen safe? And would that cause a tan? I'd like to keep my skin from tanning as a i prefer a lighter shade.
It's just too expensive to apply sunscreen 3 times a day because of 3 seperate exposer periods of just a few minutes.
Thanks :)

Neal Schultz, M.D. on August 4, 2010 at 11:30pm

@Moka: In terms of damage to the skin from the sun's harmful ultraviolet rays... being under direct sunlight for 5-10 minutes (lets say 10 minutes) a couple of times a day (lets say 3 times) without sunscreen is equivalent to being under direct sunlight for 30 minutes once a day. If that is done twice a week, then in terms of damage to the skin, it's equivalent to 1 hour without sunscreen. While you may not burn from those individual 10 minute exposures, since the cancer causing and premature aging effects of UV A rays is additive ("The meter is always running!"), you will tan since tanning merely means your skin has been damaged and tanning is the skin's way of protecting itself from additional damage. I am sensitive to the cost of sunscreen and am happy to tell you that since SPF is tightly regulated by FDA gov't oversight, you can get effective sun protection from the least expensive brand you find as long as it has an SPF of 15-30 with UV A (also known as "broad spectrum") protection.