Modern windows block UV light (if they didn’t everytime you sat indoors or sat in your car you’d get a sunburn). How much UV light do regular windows block?
Expect to find windows off-the-shelf that block more than 75% of the UV energy.
So is blocking 75% of UV light the same as an SPF 4 window? That doesn’t add up though, I know alot of people with red hair and freckles who work indoors. If that were true wouldn’t they get sunburnt after an hour of sitting indoors?
I can’t provide a cite, but my understanding was that common window glass blocks virtually ALL UV radiation (99+ %). So think of it as SPF 100+. If you want UV radiation, it is necessary to use a special glass formulation like Uviol glass.
You are looking at 99-100% UV filtering in windws, but some filters lose significant effectiveness over time. After 15 years, some lose 40-60%.
Also note that UV filters don’t block much visible light, as little as 6%, but some do block up to 40% when combined with other insulators. The human eye will start to sense darkening after about 40% light reduction.
Oh, indoor light is often indirect light. I’d imagine if they sat in a glass room with the 75% effectiveness you mention, then they’d eventually get burned. But a typical room probably isn’t a problem, as direct overhead sunlight probably is never an issue.
What is the reasoning behind UV tint then if regular windows block 99% of UV light? Are dark tinted UV windows just a scam?
As far as sitting inside, I figure if it is light out and the window is open the UV light would be hitting you, meaning a redhead sitting in his living room would be burned in about 40 minutes with an SPF 4 window.
Richard P. Feynman wrote that he was maybe the only person who actually looked at the Trinity atomic bomb test without using welder’s goggles. He figured that only UV (or X or gamma) could actually harm his eyes, so he sat in a truck and watched through the windshield.
OK, I found a cite. From Here
The site also passes on the information that it is UV-B that is most harmful. UV-A is less so, but there are some studies which suggest a link to melanomas.
So, the UV filter treatments aren’t a total scam, since some UV-A is transmitted through ordinary glass. Also, the UV-A will (even through glass) cause fading of your carpets, drapes etc. exposed to the light.
That is just so badass.
I have been doing some photographic printing using some “alternative” processes, usually processes from 19thC and early 20thC such as gum bichromate, albumen prints etc. Most of these require exposure to UV light and the unexposed materials can be handled under low wattage tungsten lamps. My experience has shown that window glass is NOT sufficient to block UV from daylight to avoid fogging materials. Also, my printing frame uses a sheet of normal glass to hold negatives and paper in good contact. This obviously must transmit sufficient UV to allow for exposure.
No, that’s wrong. Common window glass transmits almost all UV, especially UV at a frequency that damages fabrics, woods and skin.
"… *n the atmosphere’s ozone layer, 99% of the ultraviolet radiation that reaches the Earth’s surface is UVA. (Some of the UVC light is responsible for the generation of the ozone.)
Ordinary glass is transparent to UVA but is opaque to shorter wavelengths. Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.
The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air.
Carnac, your site is at the very least misleading. It implies that the atmosphere blocks 99% of all UV, and that all of that gets through window glass, which jusy isn’t true.
Those color-changing glasses respond to long-wavelength UV light. The reason they change when you go outdoors is that the long-wave UV is reaching them, something it doesn’t do when you’re indoors or in a car, because in those cases the glass blocks the long-wave UV.
Your typical borosilicate glass only transmits 8% of 309 nm light through 1 cm thickness. It starts transmitting almost all light at wavelengths above about 400 nm or so, which is about the onset of visible light. There is, nevertheless, plenty of light between 300 and 400 nm making it through the atmosphere. If there weren’t, you’d find it hard to get a tan.
I sun myself indoors all the time. What’s the SPF equivalent of a typical glass window?
Christ the work I put into this thread…
Excessive exposure to the sun even in indirect light, for example, on a cloudy day or through a glass window, may cause a serious burn
Sunlight contains the entire UV spectrum, but only about 0.5% of sunlight reaching the earth is in the UVB spectrum and at least 5% is UVA (Table 1). The intensity of UV radiation varies according to time of day and season. Maximum penetration of the ozone layer by UVB is between 10 am and 2 pm. The thickness of the ozone layer in North America varies with the seasons — thickest in late winter and thinnest in late summer and early fall. Cloud cover and air pollution have a minimal filtering effect on UV radiation. UV radiation can penetrate 1 m of water. UV rays also reflect off water, snow, sand and pavement, magnifying their effects. Window glass filters out UVB radiation but allows UVA rays to pass through. The depth of penetration of UV light into the skin is wavelength dependent. Although 90% or more of UVB photons are absorbed by the epidermis, 50% of UVA photons may penetrate to the basal layer of the epidermis or deeper. Thus, there are more potential targets for UVA photons.
http://72.14.203.104/search?q=cache:5OPg3gUrjtkJ:collection.nlc-bnc.ca/100/201/300/cdn_medical_association/cmaj/vol-160/issue-10/1471.htm+UV+transmission+"window+glass"+-film&hl=en
AMERICAN ACADEMY OF PEDIATRICS:
Ultraviolet Light: A Hazard to Children
Window glass blocks virtually all UV-B and at least half of all UV-A energy.
http://aappolicy.aappublications.org/cgi/content/full/pediatrics;104/2/328
“We can get sunburned through window glass. Normal window glass permits the transmission of a type of light called UVA.”
Transmission of UV light Though Various Materials
MATERIAL % UVA % UVB
Window glass, single thick 78 5
In the seeminly endless discussion of the spf of window glass, I noticed that someone pointed out that the use of glass in the traditional photo darkroom offered no impidiment to sunlight used in an oldfasioned process called ‘print-out printing’. It was originally done using sunlight, which happens to contaun UVA and UVB, but can be much more easily controlled using any artificial light. It just requires most any light of the visible spectrum which appears basically white to the naked eye. The UVA or UVB element is not particulary important. So it would seem that this setup (passing light through glass to expose light sensitive photo materials) offers no evidence about the blockage of UVA or UVB by the glass.
And that’s the ‘two-cents-worth’ of a 35 year photo ‘lab rat’
Joe S.
First a lot of people quoting industry sources overlooked references to “special coatings” applied to windows. That is the glazed surface does a lot of the blocking for typical residential and business windows. Just like that option for UV coating on prescription eyeglasses. However special order windows may omit that coating. Also those source quoted earlier seemed to assume that all modern windows were double paned and double glazed…which is questionable in modest priced housing.
Moreover modern windows in this context is probably mid-1970s or later. I think those UV coatings probably started phasing into the mainstream in the 1950s and 1960s but probably didn’t reach overwhelming majority until the health and eco-conscious 1970s. Good research project.
Furthermore UV radiation like all radiation is NOT blocked but is ATTENUATED. That is scientific description would state that each thickness Y of substance X causes a 50% reduction in that type of radiation transmission (measured flux). Bottomline thickness of glass matters a lot – which is why a lot of cheap UV lights can still use thin glass shell bulbs
Finally I note that apparently few people do long highway drives outside the shade of inner cities. In the middle or southern US a white person can usually get a sunburn on an arm resting against the side window of a car within 2-4 hours if driving west (sun to south) during mid-morning to mid-afternoon. Actually I’ve gotten mild sunburns within a little over 30 minutes in June-July. Because those side windows usually lack the extensive coatings that the front car windows do.
P.S. I’ve read that Feyman took several other factors into account for viewing the atomic bomb test. Even back then front truck windows where starting to get sun tinting (rather heavy handed in those early days) due the rise of long distance driving. Plus I remember accounts say he was counting on heavy reflectance due to the angle of the window with respect to the horizon. So in the end it was more about his being a brillant physicist balancing multiple factors than how many layman took the story.
UV-B protects our skin by instigating tanning. UV-A causes the skin damage if not accompanied by the protection of UV-B. So when something filters UV-B and not UV-A, watch out. There are many sites blabbing on about how certain things “filter UV” without specifying that kind of UV they are filtering. So make sure you find out. Somebody here mentioned tanning through glass. That’s a great way to shortcut yourself to an early grave. If you’ve had skin cancer or have it in your family find a device that accurately measures UV-A levels which is what we need to be wary of. “UV levels” is worse than useless. It should tell you how much UV-A and how much UV-B is present. One without the other is just brainless.
Sun safety: Get a slow, gradual tan every spring so you’re protected in the summer. Do NOT use chemical sunscreens as these chemicals are often carcinogenic. Do it naturally, do it right. Its your life so take the time to learn. Urine, if you’re eating a natural diet without processed foods, makes a miraculous skin soother if you ever burn yourself. There is ZERO smell if its fresh urine. It vastly reduces the likelihood of peeling as well. Read about Urine Therapy for some eye opening information. Martha Christy wrote a good introduction on it. There are many books and experienced experts that discuss it online. Its good for so many things. It contains zero bacteria as that has been filtered by the kidneys. It does contain over 2500 nutrients and that’s why bacteria thrive in it after a while. Bacteria are smart. 
Remember how everyone used to tell us to avoid midday sun because its strongest then? They didn’t know. In actuality its rather different. UV-A is quite consistent all day but UV-B is only at its maximum near midday. So that’s when we should be building up our gradual tan.
I did not read all the replies, but I did search for acrylic - so there are windows made of acrylic. And acrylic comes in many forms - some are UV filtering, others pass light up to 92%. I personally preferred those UV transmitting windows for my plants. UV is UVGI (ultraviolet germicidal irradiation).
@concerned 2018 - Very funny! Advising people NOT to use sunscreen and instead rub urine on themselves - hilarious. I can’t say I know of anybody dumb enough to fall for it, but good one!
Have you tried to talk anyone into using feces? It strikes me that that would be an easier sell. Same basis as the urine argument: natural, from the body, doesn’t smell bad when you first put it on etc, and it would actually block the sun’s rays better! I’m sure there’s some whack job that’s written a B.S. book supporting it too.