Understand what sunscreen labels actually mean — and how to make your sunscreen work in real life.
Book a 20 minute appointment (Sun protection plan + sunscreen advice) (automed 20 minute sunprotection appointment dr chris)
Book a full skin check (automed full skin check dr chris ivanhoe or DC)
Prefer the full overview first? (Sunscreen advice)
Key takeaways
- UVA and UVB do different damage. UVB is the main driver of sunburn; UVA penetrates deeper and is a major driver of photoageing and long-term skin damage. (1–3)
- SPF mainly measures UVB. “Broad-spectrum” is what tells you the sunscreen also has meaningful UVA coverage. (2,4)
- SPF is not linear. SPF 30 filters about ~97% of UVB, SPF 50 about ~98% — only ~1% absolute difference when applied correctly. (5)
- We usually recommend SPF 50+ not because everyone “needs” that last 1% — but because most people under-apply and under-reapply, and under-application can collapse your real SPF far more than people expect. (6–8)
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) (1–3)
Because you can’t reliably feel UV, the safest rule is to protect your skin when the UV Index is 3 or above. (9)
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 (9,10)
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 (1–3)
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. (6,7)
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 (9)
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 (5)
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. (6–8)
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. (6)
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. (6)
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!!! (6)
If a product performs as SPF 30 at the test thickness:
- at roughly half thickness, effective SPF can drop to around √30 ≈ 5!!! (6)
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. (4,7)
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 – click here If your doctor notes solar elastosis or actinic keratoses)
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 (7). 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: (9)
- 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 20 minute appointment with Dr Chris (Sun protection plan + sunscreen advice)
Or book a baseline risk check: (Full Skin Checks)
Frequently asked questions
Is SPF 30 enough?
For many people, SPF 30 applied correctly is high protection. But because under-application is common, SPF 50+ is often the simplest practical choice. (6–9)
Does higher SPF mean I can stay out longer?
Not reliably. Sweat, swimming, towelling, friction and under-application reduce protection. Reapply every 2 hours outdoors and after swimming/sweating/towel-drying. (9)
Does SPF measure UVA protection?
No — SPF is mainly about UVB. Broad-spectrum labelling is what tells you the product also meets UVA requirements. (4,7)
Why do I still tan or pigment with sunscreen?
Common reasons include under-application, not reapplying, relying on makeup SPF, missing areas (ears/scalp/neck), and high UVA exposure. For pigment-prone skin see: (Tinted sunscreen and visible light for pigmentation)
What if sunscreen always stings or causes a rash?
You may have irritant dermatitis, allergy, or photoallergy to an ingredient. Next step: (Sunscreen allergy and photoallergy: what to do)
Does sunscreen prevent skin cancer?
Sunscreen is an important pillar of prevention, especially alongside clothing, hats, shade, and skin checks. Evidence varies by cancer type and real-world behaviour. Overview: (Does sunscreen prevent skin cancer? What the evidence shows)