UVA vs UVB, SPF and broad-spectrum sunscreen explained

Understand what sunscreen labels actually mean — and how to make your sunscreen work in real life.

Prefer the full overview first? Sunscreen advice

Why UV matters (even on cool or cloudy days)

Sunlight includes visible light and invisible wavelengths. The invisible part includes ultraviolet (UV) radiation, which drives:

• DNA damage (skin cancer risk)
• collagen and elastin breakdown (premature ageing)

Because you can’t reliably feel UV, the safest rule is to protect your skin when the UV Index is 3 or above. ⁽²⁾

UVA vs UVB: the simple explanation

UVB: the “burning” rays

• The main cause of sunburn
• Strongly linked to actinic keratoses and squamous cell carcinoma risk

UVA: the “ageing + deeper damage” rays

• Makes up most of the UV reaching the ground
• Penetrates deeper and is a major driver of photoageing
• Also contributes to skin cancer risk

Practical takeaway: you want protection against both.

What SPF means (and what it doesn’t)

SPF (Sun Protection Factor) is a measure of protection against UVB when sunscreen is applied at a standard thickness in testing.

SPF does not reliably tell you:

• how much UVA protection you’re getting
• how long you can stay outdoors without reapplying
• what happens when you sweat, rub, towel-dry, or swim

That’s why we keep the advice simple:

• Apply enough
• Reapply every 2 hours outdoors
• Reapply after swimming, sweating, or towel-drying ⁽²⁾

Step-by-step method: How to apply sunscreen properly: the teaspoon rule

SPF is not linear (why SPF 30 vs SPF 50 isn’t a “huge” jump)

When applied correctly:

• SPF 30 filters about ~97% of UVB
• SPF 50 filters about ~98% of UVB

That’s only around a 1% absolute difference in UVB filtering.

So why do we still recommend SPF 50+?

Because in real life, the bigger problem isn’t whether you chose SPF 30 or SPF 50 — it’s that most people apply far less than the test thickness and don’t reapply often enough. ⁽¹⁾

Think of SPF 50+ as giving you a little more “buffer” against the very normal human tendency to underdose.

Under-application is also not linear

This is the part most people don’t realise:

The relationship between how much sunscreen you apply and the protection you get is not linear. Evidence suggests SPF drops in an exponential way with thinner application — often approximated by a “root” relationship. ⁽¹⁾

A useful rule-of-thumb from published research is:

If you apply half the tested amount, your achieved SPF can fall to roughly the square root of the labelled SPF. ⁽¹⁾

What that means in practice

If a product performs as SPF 50 at the test thickness:

• at roughly half thickness, effective SPF can drop to around √50 ≈ 7. ⁽¹⁾

If a product performs as SPF 30 at the test thickness:

• at roughly half thickness, effective SPF can drop to around √30 ≈ 5. ⁽¹⁾

So yes — it is often better to use SPF 50+ because when you under-apply (as most people do), you’re more likely to still land in a “better” protection range than if you started at SPF 30. The real win is still applying and reapplying properly.

If you want the practical amounts, not the maths: How to apply sunscreen properly: the teaspoon rule

What “broad-spectrum” means in Australia

Broad-spectrum means the sunscreen meets requirements for UVA and UVB protection — not just UVB.

Practical takeaway: for daily use in Australia, look for:

• SPF 50+
• Broad-spectrum
• A finish/feel you will actually use every day

If sunscreen stings or flares your skin: Sunscreen for rosacea, acne, eczema and pigment

If you want specific advice on which sunscreens Dr Chris recommends, see our main sunscreen guide. If your doctor notes solar elastosis or actinic keratoses, see actinic keratosis treatment.

Water resistance: what it does (and what it doesn’t)

Water resistance means a sunscreen has met a standard test after water exposure for a defined period. The defined period depends on the specific SPF rating, but is generally between 40 minutes - 4 hours. I recommend all my patients reapply sunscreen every hour while swimming or at the beach and getting in and out of the water (and remember that sunscreens take around 20 minutes to absorb into the skin - don’t apply the sunscreen and then run straight back in! You’re wasting it!)

It does not mean:

• you can skip reapplication
• the sunscreen is “waterproof”
• you’re protected indefinitely during sweat, friction, towelling, or swimming

If you work outdoors or train hard: Sunscreen for outdoor workers and athletes

A simple, reliable sunscreen checklist

When the UV Index is 3 or above: ⁽²⁾

• Choose SPF 50+ broad-spectrum
• Apply before sun exposure
• Reapply every 2 hours outdoors, or every hour when getting wet
• Reapply after swimming/sweating/towel-drying
• Add hat, clothing, sunglasses, and shade

More than sunscreen: UPF clothing, hats, sunglasses and shade

Book a personalised plan

If you’re outdoors a lot, burn easily, have pigment issues, or want a simple routine you’ll actually follow, book a baseline risk check: