If you've ever looked at a sunscreen label and wondered what "broad-spectrum" actually means — this is the article for you. The term refers to protection against two distinct types of ultraviolet radiation: UVA and UVB. They're both invisible, both damaging, and both present whenever you're outdoors. But they behave differently, penetrate your skin differently, and cause different kinds of harm.
Understanding the difference is one of the most practically useful things you can know as an outdoor enthusiast. Here's the science, made plain.
The UV Spectrum: A Quick Overview
Ultraviolet radiation is part of the electromagnetic spectrum, sitting just beyond visible light at wavelengths shorter than 400 nanometres. The sun emits three types — UVA, UVB, and UVC — but UVC is almost entirely absorbed by the Earth's atmosphere and doesn't reach the surface. Source: WHO Global Solar UV Index, 2002.
That leaves UVA and UVB as the two types that reach your skin every time you step outside. They're both damaging, but in meaningfully different ways.
UVA: wavelength 315–400nm. Accounts for up to approximately 95% of UV radiation reaching Earth's surface. Present year-round, at relatively consistent intensity throughout the day. Penetrates cloud cover and glass. Source: Skin Cancer Foundation; IARC Monograph Vol. 100D.
UVB: wavelength 280–315nm. Accounts for approximately 5% of UV radiation at ground level. Intensity varies significantly by season, time of day, altitude, and latitude. Mostly blocked by glass. Source: IARC Monograph Vol. 100D; WHO Global Solar UV Index, 2002.
Source: IARC Monographs Vol. 100D; WHO Global Solar UV Index, 2002.
What UVA Radiation Does to Your Skin
UVA rays have a longer wavelength than UVB, which means they penetrate deeper into the skin — reaching the dermis, the deeper layer beneath the surface. This deeper penetration is what makes UVA particularly insidious: the damage accumulates over years without producing the obvious, immediate warning signal of sunburn.
UVA is the primary driver of photoageing — the wrinkles, sunspots, loss of elasticity, and leathery skin texture associated with long-term sun exposure. It does this by damaging collagen fibres and generating free radicals in the deeper layers of skin. Source: Skotarczak K et al., Advances in Dermatology and Allergology, 2015.
More seriously, UVA also contributes to skin cancer risk, including melanoma risk. It damages DNA in skin cells, and while UVB-related sunburn is also strongly linked to melanoma, UVA's contribution to skin cancer is well established. Source: IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 100D, 2012; Skin Cancer Foundation.
The other critical characteristic of UVA: it can penetrate ordinary window glass. This means you may receive UVA exposure while driving, sitting by a window, or working near glass — though UV-blocking films or treated glass can significantly reduce this. Most people assume they're protected in these situations, but ordinary glass offers little UVA defence.
What UVB Radiation Does to Your Skin
UVB rays have a shorter wavelength and are largely absorbed in the outer layer of the skin — the epidermis. This is where they cause sunburn: the redness, pain, and peeling that follows overexposure. UVB is the primary cause of sunburn and the most direct driver of DNA damage in skin cells.
That DNA damage is what links UVB exposure to skin cancer, particularly squamous cell carcinoma and basal cell carcinoma — the two most common types of skin cancer. UVB is also the primary driver of actinic keratosis, the precancerous skin lesions that are a significant risk for outdoor enthusiasts with cumulative sun exposure. Source: AAD Guidelines on Melanoma and Non-Melanoma Skin Cancer, 2023; IARC Monograph Vol. 100D.
UVB intensity changes significantly with the season, time of day, altitude, latitude, and ozone conditions. It's highest between 10am and 4pm, in summer, at high altitude, and near the equator.
UVA varies less dramatically than UVB, but it still changes with time of day, season, latitude, altitude, and cloud conditions. This is why a low UV Index doesn't mean zero UVA exposure — meaningful UVA can still occur outside peak summer conditions.
Unlike UVA, UVB is mostly blocked by glass. However, cloud cover does not eliminate UV exposure — a significant proportion of UV radiation can still reach the skin on overcast days. Sun protection remains relevant even when it does not feel sunny. Source: WHO Global Solar UV Index, 2002; AAD.
UVA vs UVB: Side by Side
| UVA (315–400nm) | UVB (280–315nm) | |
|---|---|---|
| % of UV at surface | ~95% | ~5% |
| Skin penetration | Deep (dermis) | Superficial (epidermis) |
| Primary damage | Photoageing, collagen damage | Sunburn, direct DNA damage |
| Cancer risk | Melanoma (significant contributor) | BCC, SCC, melanoma |
| Seasonal variation | Low — relatively constant year-round | High — peaks in summer, midday |
| Passes through glass? | Yes | Mostly no |
| Passes through clouds? | Yes (up to 80%) | Yes (up to 80%) |
| Visible warning sign? | No — damage is silent | Yes — causes sunburn |
Sources: WHO Global Solar UV Index, 2002; IARC Monograph Vol. 100D; AAD, 2023.
Why Both Matter for Outdoor Enthusiasts
The practical implication of understanding UVA and UVB is simple: you need protection against both, every time you're outdoors.
Many people only reach for sunscreen when the UV Index is high, or when they're spending extended time in direct sun. But because UVA is present at consistent levels year-round — including on overcast winter days, including at low UV Index readings — cumulative UVA exposure builds up even on days when you'd never think to apply sunscreen.
For hikers, trail runners, cyclists, and skiers, the stakes are higher. Extended time outdoors at altitude, near reflective surfaces, or during peak hours significantly increases both UVA and UVB exposure simultaneously. Source: WHO Global Solar UV Index, 2002; Diffey BL, Methods, 2002.
A sunscreen labelled broad-spectrum protects against both UVA and UVB. In the EU, a product can only carry the broad-spectrum label if its UVA protection factor is at least one-third of the SPF value.
The SPF number on a bottle measures UVB protection only. Without the broad-spectrum label, you have no indication of how well — or whether — a product protects against UVA.
Always choose broad-spectrum SPF 30+ as your minimum. For extended outdoor activities, broad-spectrum SPF 50+.
Source: EU Cosmetics Regulation (EC) No 1223/2009; AAD Sunscreen Guidelines, 2023.
How to Protect Against Both UVA and UVB
Choose broad-spectrum sunscreen. This is the single most important step. Look for "broad-spectrum" on the label and SPF 30+ as your minimum — SPF 50+ for extended outdoor activities. Apply it 20–30 minutes before going outside and reapply every two hours.
Wear UPF-rated clothing. UPF (Ultraviolet Protection Factor) rated clothing blocks both UVA and UVB. UPF 50+ fabric blocks approximately 98% of UV radiation when properly worn and not significantly stretched or wet beyond testing conditions. Source: ARPANSA, Australian Radiation Protection and Nuclear Safety Agency. For hikers, trail runners, and cyclists who spend hours outdoors, UPF clothing is more reliable than sunscreen because it doesn't degrade, sweat off, or need reapplication. Source: ARPANSA, Australian Radiation Protection and Nuclear Safety Agency.
Wear UV400 sunglasses. Your eyes are exposed to both UVA and UVB. Long-term UV exposure to the eyes is a significant risk factor for cataracts and macular degeneration. UV400 lenses block all wavelengths up to 400nm — covering both UVA and UVB completely. Source: WHO, Prevention of Blindness and Deafness, 2011.
Don't rely solely on the UV Index. The UV Index is weighted toward sunburn-producing radiation and is especially influenced by UVB, but it represents erythemally weighted UV radiation rather than UVB alone. Importantly, a low UV Index does not mean zero UVA exposure. On overcast winter days, UVA exposure is still occurring — which is why dermatologists recommend daily broad-spectrum SPF application as a baseline habit, not just a warm-weather practice.
The Bottom Line
UVA and UVB are two different types of radiation causing two different patterns of damage — but both contribute to skin cancer risk and both require protection. UVB causes the immediate, visible damage of sunburn. UVA causes the slower, silent damage that accumulates over years and drives photoageing and deeper skin changes.
The good news is that protecting against both is straightforward: broad-spectrum sunscreen, UPF clothing, UV400 sunglasses, and awareness that UV exposure happens year-round — not just on sunny summer days.