Do UV Sun Rays Go Through Windows?

The answer is yes, usually some important UV rays do come through window glass.

Recently a reader sent me this question as a comment to one of my blog posts.  It’s such a great question, and one patients often ask me, so I decided to make an Ask Dr. Bailey post out.

You may think that you don’t need to worry about sun damage when you’re indoors, but that’s not true if you’re sitting in front of a window…..

John asked:

Hi Dr. Bailey,

I work in a “trendy” office with floor to ceiling windows on all the outside walls. I know many commercial buildings now have UV window film that appears untinted to the human eye but block almost all UV radiation. Is there any way to tell if the window next to me has this type of filtering in place? It would save me from reapplying sunscreen at my desk every 3-4 hours!

Best Regards and love the blog!


Hello John,

This is a really important issue for anyone sitting in direct sunlight that comes through a window. Normal glass blocks all of UVB but allows UVA to come through. Both of these rays are harmful.  Remember:


  • UVB is considered the main sunburn ray
  • It causes skin cancer and sun damage
  • The SPF in a sunscreen tells you how well the product protects you from UVB


  • Is the UV ray in tanning beds
  • It penetrates the skin more deeply than UVB playing a big role in sun damage including wrinkles
  • It can cause skin cancer
  • The SPF on sunscreens tells you nothing about whether the product protects you from UVA.  Look for sunscreens labeled ‘broad spectrum’, but even then full UVA protection is more complex.  Bottom line, I recommend mineral sunscreens for the best, most reliable UVA protection (click here to read more) or Mexoryl SX if you really don’t like mineral sunscreens.

As you point out, some modern windows block most all of the UV rays including UVA. Finding out which windows were used in the construction of your building might help answer that question. It’s possible to block up to 99.9% of UV waves with today’s modern technology. The Skin Cancer Foundation has information on window UV film, click here to see it. In general, you can add sheets of UV blocking film to windows, or purchase glass that includes technology to block UV rays.

The Skin Cancer Foundation also has a list of window films that they have investigated and whose reliability they approve of.  Click here to read more. They also have a a glass manufacture that’s passed their muster too, click here see it. Beyond that, the only way to accurately test how much UV passes through your windows would be to finding someone with a radiometer that measures the UV transmittance.

For my patients who sit in direct sunlight coming in through window glass, I recommend wearing a mineral sunscreen and clothing that blocks UV rays (click here to read about sun protective clothing). If their windows claim to block UV rays I point out that a very tiny amount of UV still gets through.  As I mentioned above, it’s possible to block 99.9% of UV rays, but even if their windows are that good, the 0.1% can add up at 8 hours a day, 5 days a week.  Wrinkles, age spots and skin cancer just ain’t worth it!

For exposure through windows that don’t claim to block UV rays I definitely tell my patients to reapplying mineral sunscreen every 2 hours if they can’t move out of the direct sunlight.  It’s also time to get creative about trying to create shade too. That would be pretty tough in a ‘trendy’ office building with floor to ceiling windows, which is what makes this question such a challenge.

You’ve asked an excellent and important question. Best of luck.

Warm Regards,

Cynthia Bailey MD, Dermatologist

If you have questions about skin care or skin health please send them to me using the Contact Dr. Bailey button at the top of the page.

Disclaimer: Please realize that availing yourself of the opportunity to submit and receive answers to your questions from Dr. Bailey does not confer a doctor/patient relationship with Dr. Bailey. The information provided by Dr. Bailey is general health information inspired by your question. It should not be a substitute for obtaining medical advice from your physician and is not intended to diagnose or treat any specific medical problem (and is not an extension of the care Dr. Bailey has provided in her office for existing patients of her practice). Never ignore your own doctor’s advice because of something you read here; this information is for general informational purpose only.

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17 Responses to “Do UV Sun Rays Go Through Windows?”

  1. Rae Aben August 2, 2011 at 9:30 am #

    How about sitting in front of a computer screen the whole day. Does a laptop/ computer screen ’cause UVA/ UVB damage?

  2. Kathy Krause August 2, 2011 at 11:26 am #

    Hi Dr. Bailey: I have a window in my closet (old house) and the sun is fading my clothes. You just saved me a whole lot of research on what film to use on the window. Thanks for the links. You always help me in more than one way!!! Thanks–Kathy

  3. Cura Pelle August 3, 2011 at 10:41 am #

    Do mineral sunscreens also break down? Are they not photo-stable? I had thought that they were, hence the justification for not reapplying every couple of hours. False?

  4. Cynthia Bailey MD August 4, 2011 at 7:59 am #

    Hello Rae,
    That’s hard to know for sure. From what I can tell some do emit a little UV, and some don’t. It looks like it’s a function of the technology used to construct the screen and that the newer ones are less likely to emit UV, but I could not find any conclusive analysis on the subject. Again, the only way to know if a particular computer screen is emitting any UV rays would be to find someone with a radiometer and have the screen tested. For people with highly UV sensitive skin conditions, such as xeroderma pigmentosa or lupus, this may necessary.
    Cynthia Bailey MD

  5. Anson Ng August 4, 2011 at 8:56 am #

    Dear Dr. Bailey,
    I just want to comment on your recent blog post on the new FDA sunscreen rules.
    1) Isn’t the capping of SPF values at SPF 50+ a proposed rule rather than a final rule?

    2) I thought titanium dioxide protects up to 350nm, Mexoryl SX protects up to 390nm, and avobenzone protects up to 400nm? I’m just quoting the following article from the April Issue of the JAAD.

    Sambandan, Divya R., and Desiree Ratner. “Sunscreens: An Overview and Update.” Journal of the American Academy of Dermatology 64.4 (2011): 748-58. Print.

    You’re probably quoting other literature?

    3) I do understand that inorganic filters are much more photostable, but I have come across some articles showing that organic filters provide a higher PPD value than inorganic filters? Dr. Baumann also says in a recent post on her blog that “chemical sunscreen ingredients offer the best protection, especially against cancer- and age-causing UVA rays.”

    What’s your take on this issue?

    I would love to know what you think. Thank you so much!

  6. Cynthia Bailey MD August 4, 2011 at 6:42 pm #

    Hello Anson,

    I love your question and you have great reading material! My opinion is based on a lot of scientific literature over the years, medical symposia, conversations with sunscreen formulating chemists and my clinical observations watching people use a variety of sun protection products. Interestingly, I can’t find my April JAAD, indicating I must have been reading it somewhere and need to back track and figure out where I left it.

    My clinical experience is my daily reminder of what works and doesn’t. I do at least 7 to 10 full skin exams every day and tons more partial exams. When I see sunscreen product failure it’s almost always (quite possibly always always) the chemical sunscreens. They just don’t protect reliably when used by consumers. Add to that the complexity of creating a stable, reliable sunscreen recipe and I just don’t gamble. I recommend the products I trust, that I’ve seen people use with almost 100% success (failure being limited to reapplication issues) and I stick with them. They are nano zinc and octinoxate (omc), which is a cleaver almost magical combo. Remember the old Z-Cote Mitchnick, Fairhurst and Pinnell article in JAAD in 1999? They built the foundation and formulation chemists have been improving of recipes ever since. A table in Wang, Photoprotection: a Review of the Current and Future Technologies in Dermatologic Therapy, Vol. 23, 2010, page 36 has different absorption peaks than you mention. Of course, all the ingredients trail off as they reach 400, zinc ox included.

    My conclusion is that sunscreen formulation science is still in evolution and there is no obvious guide post. Therefore my clinical observations factor heavily in my sunscreen recommendations. I and most of my patients are skin types 1 to 3 living in the sun belt and outside all year. We are fantastic guinea pigs and we do best with the zinc and/or zinc/omc combo.

    Thanks for doing your homework!

    Warm Regards,
    Cynthia Bailey MD

  7. Anson Ng August 4, 2011 at 8:33 pm #

    Dear Dr. Bailey,

    Thank you so so much!! I really appreciate your answers and I hope I didn’t sound too intimidating :P I guess that explains why my dermatologists often recommend physical filters over chemcial ones. I’m a guy and physical sunscreens make people think that I’m wearing a foundation. But the chemical ones with SUNSPHERES techonology (, like most of Neutrogena’s sunscreens and some of the Anthelios ones, also leave a white film on the skin. Oh well :(

  8. Anson Ng August 4, 2011 at 9:16 pm #

    Could you please just answer one more question because I feel like my dermatologist doesn’t have the time or isn’t too familiar with sunscreen formulations :P

    What’s your opinion on the effectiveness of sunscreens with zinc oxide (or with mainly physical filters) compared with that of European (specifically Loreal) sunscreens with titanium dioxide, Mexoryl XL, Mexoryl SX, Tinosorb S, and avobenzone such as the Anthelios sunscreens, some of which boast a PPD of 42. (

    Similar to the case of SPF, I don’t know how much clinical significance such a high PPD value carries. Even though such sunscreens are not yet available in the US, many people who are sun-conscious or have photosensitivity are buying them online or from dermatolgist’s offices. My doctor actually carries them.

    Or maybe the best sunscreen is one that is applied adequately and reapplied frequently :P

    Thank you so much! :)

  9. Cynthia Bailey MD August 5, 2011 at 6:05 am #

    Hello again,
    Zinc oxide clumps together so there is a trick to getting a pretty clear application. I tell my patients to first squeeze product into one hand on the palmar aspect of the fingers. Then rub the fingers of both hands together briskly to warm the product and begin to layer the particles out singly. Then apply it to the skin and rub some more until it appears clear (or clearer).

    I think Mexoryl SX probably provides reliable protection based on the literature and a very small number of my patients that have bought it online. I’m not a titanium dioxide fan for many reasons and I’m not a fan of avobenzone. I have no experience yet with Tinosorb products. I don’t carry products not approved by the FDA and will wait for them on this one. As I’ve mentioned, I’m conservative with sun protection recommendations and am so specific as to stick with products I have seen perform successfully numerous times. The ‘devil is in the details’ of how a product is made and I stick with chemists and labs that have proven to me that their products are 100% reliable, patient after patient. Of course, sunscreen is no substitute for hats, protective clothing and shade.

    Warm Regards,
    Cynthia Bailey MD

  10. Cura Pelle August 5, 2011 at 12:26 pm #

    This has been a busy thread! If it’s not too much trouble, could you provide a clarification on whether or not mineral sunscreens need to be reapplied every couple of hours?

  11. Cynthia Bailey MD August 5, 2011 at 4:01 pm #

    Hello there,

    Well, I have to say yes because that’s the recommendations mandatory on all sunscreen products. That said, zinc oxide is used in industrial applications as a sun protective coating with a long life. It may well last much longer on the skin. Most sunscreen products though are a combo of zinc oxide and some chemical UVB blocking active(s) like octinoxate because zinc oxide usually can’t give an SPF 30 all by itself without being white on the skin. These other actives will break down for sure with UV. Add to that the fact that the specific product formulation itself effects ingredient photostability and every product would to have it’s photodegredation data determined individually. You can see that it’s impossible to make broad statements based on a single active.

    With sunscreens you can always tell if the product or your application practice is working because if you put it on inadequately it doesn’t give you the protection you expect. Any tan, freckle darkening or, heaven forbid, sunburn is a big message. The zinc probably does cut us a little slack though, which is one of the thing I love about it, and why I call it the most ‘reliable’ and ‘trustworthy’ sunscreen ingredient in my opinion.

    Warm Regards,
    Cynthia Bailey MD

  12. Cura Pelle August 6, 2011 at 9:35 am #

    Thank you for another good column and also for addressing my question.

  13. tanless August 12, 2011 at 10:18 am #

    In my experience Tinosorbs (S and M) are amazing. Tinosorb M is one of the two filters with better long UVA absorbtion than zinc oxide. 90% of EU drugstore sunscreens contain both. Standard combination is TiO2, Octocrylene and Avobenzone and both Tinosorb S and M. It’s not a perfect solution, but it works and is reasonably photo-stable. Other EU sunscreens contain another little miracle called Uvinul A Plus, the other filter that has wonderful UVA efficiency, very stable, non-reactive and effective in small amounts. When I used sunscreen with it in I would not get any color on a beach, I just didn’t care for the UVB filter combination in that particular formula.
    Sadly, ZnO is still not approved as sunscreen filter in EU, so it’s very expensive to import them from USA all the time. Also I had to find out the hard way that not all zinc sunscreens are equal, some are actually horrible at protecting skin, as in, they are less effective than TiO2 and do not protect against long-UVA rays.
    It’s always a hard choice for me: Tinosorb M or Uvinul A Plus which are both amazing for me, but are usually paired with Avobenzone (unstable) and Octrocrylene (free radical concerns) OR ZnO which is whitening and costs me 3x more. Tough.
    In a perfect world I’d use a sunscreen with ZnO such as Z-Cote with both Tinosorb M and Uvinul A Plus, all three of them fully encapsulated.

  14. Cynthia Bailey MD August 15, 2011 at 6:44 am #

    Thanks! This is great experience and I look forward to getting to try the Tinosorb products, hello FDA, anyone listening in there???? I also love your comment about not all zinc ox sunscreens being equal, you are so right. The ‘how’ is an important as the ‘what’ with sunscreen formulation. It’s why a person shouldn’t go buy Brand X just because the price is tempting. Thanks again, CBMD

  15. Vivienne August 19, 2011 at 9:55 pm #

    Hi Dr. Bailey,

    I was wondering if I spend the day inside my home, with the window blinds or curtains down and a small amount of light still leaking through (enough light so that the room is not dark), is it necessary to wear sunscreen? Are UVA rays leaking through and am I being exposed to them? Thanks.

  16. Cynthia Bailey MD August 20, 2011 at 7:57 am #

    Hello Vivienne,
    In a purest, scientific sense it may be possible. For a person who is exquisitely sensitive to UVA rays such as someone on light sensitizing medicine then this could possibly still trigger a reaction. I always recommend applying sunscreen in the morning when we wash our face for the day. One always hopes that the day will call us outside and into nature, to run an errand or to socialize and then we are covered.
    Warm Regards,
    Cynthia Bailey MD

  17. Vivienne August 20, 2011 at 10:22 am #

    Thank you Dr. Bailey. :)