“UVA rays make up roughly 95% of UV radiation reaching Earth's surface, penetrate deeper into the skin (the dermis) than UVB, and are primary contributors to tanning, hyperpigmentation, and photoaging”

The 95% figure is not folklore. WHO states that the relatively long-wavelength UVA accounts for approximately 95 per cent of the UV radiation reaching the Earth's surface, that it can penetrate into the deeper layers of the skin, and that it is responsible for the immediate tanning effect. WHO also explains that as sunlight passes through the atmosphere all UVC and most UVB is absorbed by ozone, water vapour, oxygen and carbon dioxide, while UVA is not filtered as significantly. IARC's monograph on solar radiation reports the same split, noting that approximately 5% of solar terrestrial radiation is ultraviolet radiation, and solar radiation is the major source of human exposure to it, with the ultraviolet portion itself being roughly 95% UVA. On penetration depth, the dermatology literature is consistent. UVA penetrates deeply into the dermis, causing oxidative stress and collagen breakdown, while UVB primarily affects the epidermis, inducing DNA damage and inflammation; both mechanisms activate matrix metalloproteinases, the enzymes responsible for collagen degradation. A review of long-wave UVA reports that UVA rays can pass through the epidermis and reach the lower dermis, and that approximately 100 times more UVA than UVB photons reach the dermis. Another review notes that even though UVA is less energetic than UVB, it is more abundant and penetrates deeper into the skin, reaching as far as the dermis. On pigmentation the picture is split by mechanism. For immediate and persistent darkening, UVA dominates: UVA radiation induces immediate pigment darkening and delayed tanning, while UVB radiation generates only delayed tanning. But for the melanin-synthesis tan, UVB is the more efficient waveband. UVB is 3 to 4 orders of magnitude more effective per unit physical dose than UVA for DNA photodamage, erythema, tanning and skin cancer in mice. Exposure to UVB causes erythema and delayed tanning, and UVB is more efficient than UVA in causing these effects. A comparison of UVA with UVB in eliciting increases in skin colour found UVB more effective at stimulating skin pigmentation than UVA, though both worked. UVA can produce pigment darkening lasting multiple months, which some authors describe as a UVA delayed tanning via neo-melanogenesis, but other studies show significantly less to no production of eumelanin, pheomelanin and melanogenic intermediates with UVA versus UVB exposure, and suggest UVA-induced tanning may not confer the photoprotective benefit UVB-induced tanning does. For hyperpigmentation specifically, long-wave UVA plus visible light is the relevant band, and the effect is strongest in deeper skin tones. Long-lasting and sustained skin pigmentation has been reported in subjects with skin phototypes IV to VI but not in fair-skinned individuals with phototype II. A review of pigmentation disorders describes a synergistic relationship between UVA1 and visible light that intensifies induced pigmentation, and concludes that conventional sun protection focused only on UVB and shorter-wavelength UVA is not sufficient to prevent or control pigmentation disorders. The 95% is an average, not a constant. The UVA to UVB ratio is about 17 at a solar zenith angle of 20 degrees and is almost constant up to 60 degrees, after which it rapidly increases because UVB wavelengths are more attenuated than UVA. The Skin Cancer Foundation phrases the same figure as UVA accounting for up to 95 percent of the UV radiation reaching the earth, and notes these rays maintain the same level of strength during daylight hours throughout the year.