Most airplanes fly at >30k feet, way into the stratosphere / mesosphere where sun's UV rays aren't filtered by lower atmosphere layers.
If the windows of the plane are open and the sun is shinning right on a passenger, will the UV rays be more harmful than those on the earth's surface? Are there any long term effects if a passenger is exposed frequently?
Hmmm, opening the window at 30.000ft, where the air temperature is about -40°C/-40°F, air speed is extreme, you can't survive without oxygen, and you want to put your arm outside???
Well, if you really do, you'll get a sunburn quite fast. I got a sun burn within an hour in an altitude of 3500m / 12000ft, though I was tanned due to two weeks at the beach.
The ozone did already absorb a large fraction of the UV-C radiation, but as most of the air mass is already below you, the rest of the sun light will not have been filtered so much.
So, let's close the window.
To my knowledge, side windows and canopies are made of some kind of polycarbonate like or Plexiglass.
CERN measured the transmission of light through a 2mm sheet of plexigas and came to the result that no light below ~350nm, passes this sheet. Depending on source, the UV region starts below 380-400nm.
Source: https://cds.cern.ch
Keep in mind that the windows are usually thicker, which makes this step in the diagram much steeper.
So, there isn't much UV left which can cause a sun burn, but let's take the data from rredc.nrel.gov to calculate how much light you get and compare it to the light at ground. I found a function describing the data from CERN quite well, and applied it to the sun spectrum data (without atmosphere) twice, to get a Plexiglass window of 4mm thickness. So, this is the worst-case, with no air above the aircraft pre-filtering the sunlight.
Here is the result, which shows that sunlight is much more intense, and its color is heavily shifted to blue. But the intensity drops to zero, just where the UV-A range starts. Even, if you put the beginning of the UV-A range to 400nm, the intensity is still lower than at the ground.
Wind shields of larger aircraft are usually a quite thick composite material of glass and some plastics, which should absorb UV the same way as polycarbonate does, if not better.
So, I don't see any evidence that you get more UV light in an aircraft than on ground.
However, you should not underestimate cosmic radiation, which is absorbed by our atmosphere quite well on the ground, but not at high altitude. A transatlantic flight is equal to an X-ray photography of your chest. Flying staff is an occupation group exposed to much higher radiation doses than any other group and it's dose usually is supervised.
EDIT:
As @PeterKämpf pointed out, Plexiglass is only one material used, and it is not a polycarbonate, which I didn't notice. But polycarbonates don't differ much concerning UV transmission, as the following diagram shows. However, I also read that polycarbonates - in contrast to plexiglass - are not UV hard and need an UV blocking coating when used outside.
Source: www.plasticgenius.com
You're right. According to the WHO1 at 30,000+ feet, UV rays are more intense. They're around 90% more intense, which is enough to cause some pretty nasty skin, and eye damage.
Pilot's do have a higher risk of skin cancer2. Although, the jury is out on why this is the case. Most cockpits themselves are made of a different set of materials.
The FAA itself does not have any requirement3 specifically on the opacity to UV rays for windows. Though, they point out that typically passenger cabin windows have a protective layer, and typically use some acrylic or polycarbonate. The regulations themselves state the the material must be tolerant to UV exposure itself. Polycarbonate4 absorbs most of the UV spectrum. Acrylic on the other hand absorbs UVB, and UVC, the most harmful type of UV radiation5. It, unfortunately, allows some component of the UVA portion of the through (less than 50%6). On top of this, most airlines glaze their windows, blocking even more light.
Some modern aircraft utilize electronics in the windows to dim them. The materials that are involved in this allow for better filtering of light, and in addition, allow for the cabin crew to dim the entire cabin in high light situations7.
In conclusion, you're a pilot, there may be some higher risk in exposure to UV rays, and you should take appropriate precautions, such as wearing sunscreen. If you're a passenger that flies often, it's probably okay, and no worse than taking a walk in the sun, but if you have very sensitive skin, wearing sunscreen will be an effective way to prevent the little damage that can occur. Fortunately, airplanes are getting more advanced every day, and the risks are becoming fewer, and fewer.
Sources
As per Wikipedia, the ozone layer is from approximately 20 to 30 kilometres (12 to 19 mi) above Earth. 30,000 feet is 9.1 km which is below the ozone layer, hence the UV rays passing below this range would have been filtered already.
There are some side effects on skin (cancer) and eye - refer for more details. If this is causing severe problems, then FAA/FAR would have had the regulations to fly below a certain altitude. I don't think there will be any severe effects on the skin as well on the eyes from the UV rays below the ozone layer.
Also per FAA Safety, all civil aircraft cannot cross Class A (flying under IFR) airspace cannot exceed FL 600 (~60k ft) which is almost below the ozone layer, so no to the question of exposing to direct UV under these altitudes.
UV exposure supposedly increases aout 4% every 1,000 feet so folks living in Colorado would get about 25% more UV than people living at sea level. However, some of these comments appear to say all UV exposures below the ozone layer (60,000 feet to 100,000) are equal. Hmm... I have always heard that airline personnel have a high risk because of spending so much time at 30,000 feet plus. I don't have a technical background but I do know that ozone blocks UVB more than UVA and the air itself may absorb some as well.
The material of the windows will filter most or all of the UV radiation from the incoming light. That's the reason as well why you don't get a tan driving your car with the windows rolled up (yes, car windows are made of different materials, but the effect is the same).
Yes, the level of UV radiation at high altitude is more intense than at low altitude, which is why wearing eye protection and protecting the skin against exposure at high altitude is important.
But unless you were to fly at 30.000 feet in an open aircraft without windows there's no reason to worry about that when flying. And if your window were to blow out at that altitude you have more serious problems than getting a bit of a sunburn :)
