UVA and UVB: which one is doing what to your skin
One burns, one ages, and only one of them is stopped by your window. A plain guide to the two bands of ultraviolet that reach you.
Short answer: UVB is the burning band and it is largely blocked by glass. UVA passes through glass and cloud, reaches deeper, and is the band most associated with visible ageing. A sunscreen needs to answer both.
UVB — the one you feel
UVB is the shorter wavelength and the one behind sunburn. Because it is what reddens skin, it is what SPF measures. It is strongest in the middle of the day and it is substantially reduced by ordinary window glass — which is why you can sit by a window all afternoon without burning, and conclude wrongly that you were protected.
UVA — the one you do not
UVA is the longer wavelength. It penetrates further into the skin, it is present across daylight hours, and per WHO INTERSUN guidance ordinary window glass transmits it. You will not feel it working. In clinical photoageing research, up to 80% of visible facial ageing has been attributed to UV exposure rather than to the passage of time (Flament et al., 2013).
Because you cannot feel UVA, the indoor desk and the car commute are the exposures people discount most and should discount least.
How each is measured
SPF, the UVB measure, is tested under ISO 24444. UVA protection is tested separately under ISO 24442, and that is what the PA rating on a pack reports. They are two different tests. A pack that quotes one and stays quiet about the other has answered half the question.
What this means for a daily routine
Daily, not seasonal. Across most of India the WHO UV index sits at high or above for most of the year, December included. If a product covers both bands, is comfortable enough that you will actually reapply it, and states what it has been tested for, it is doing its job.
Our gel is rated for both bands on the pack; what has and has not been measured for each production batch is published on the proof page.
