UV protection from windows

There are a two questions I want to address in this posting:

1. How much UV radiation to normal windows block?

Normal glass is good at filtering out lower wavelength UV waves (UVB), but UVA waves predominantly transmit through the glass.

2. Do the UV films and special UV windows actually make a difference?

In this study, the UV film added to the auto glass had a significant reduction in cytotoxicity compared to normal glass. The plain glass offered 29% protection while glass and film together offered 93% protection. This includes longer wavelength UVA. But this study only measured the resultant effects on cells and did not actually measure which and how much UV radiation went through the glass, making it difficult to draw conclusions about UVA v. UVB protection from the film.

Bernstein EF, Schwartz M, Viehmeyer R, Arocena MS, Sambuco CP, Ksenzenko SM. Measurement of protection afforded by ultraviolet-absorbing window film using an in vitro model of photodamage.Lasers Surg Med. 2006 Apr;38(4):337-42.

The Skin Cancer Foundation has a list of recommended window films for UV protection. I haven't been able to figure out how they chose these window films. On 3M's website, they say that all of their films listed here block 99.90% of UV radiation, but again, no details.

According to

Edlich RF, Winters KL, Cox MJ, Becker DG, Horowitz JH, Nichter LS, Britt LD, Long WB, Edlic EC.Use of UV-protective windows and window films to aid in the prevention of skin cancer. J Long Term Eff Med Implants. 2004;14(5):415-30.

the International Window Film Association offers certifications for certain levels of UV protection, but after checking out their website, I couldn't find any detailed standards.

And this article

J.A. Johnson and R.M. Fusaro, Broad-spectrum photoprotection: the roles of tinted auto windows, sunscreens and browning agents in the diagnosis and treatment of photosensitivity, Dermatology 185 (1992), pp. 237–241.

is able to correlate visible light transmitted to UV light transmitted. Basically concluding that the less visible light transmitted, the broader spectrum of UV protection available.

Basically it's hard to tell which window tints are better than others, but it is safe to assume that darker tints will block more UV radiation.