Question 1

What do SPF numbers actually mean, and how should sunscreen be used?

Accepted Answer

SPF measures protection against **UVB only** — the radiation that causes sunburn — and the number is a ratio rather than a percentage or a time, which is the source of almost every misunderstanding about it.

**What SPF actually is.** The ratio of UV needed to burn protected skin against unprotected skin. **SPF 30 means it takes 30 times as much UVB to produce the same redness**, under laboratory conditions with a thick, even application.

**Why the numbers are less different than they look:**

**SPF 15 blocks roughly 93% of UVB.**

**SPF 30 blocks roughly 97%.**

**SPF 50 blocks roughly 98%.**

**The difference between 30 and 50 is small in blocking terms**, and meaningful mainly because it provides margin for the fact that nobody applies enough.

**The application problem, which matters more than the number.** The laboratory figure assumes **2 mg per square centimetre** — roughly a teaspoon for the face and neck, and a shot glass for the body. Real-world application is typically **a quarter to half of that**, and protection falls disproportionately, not proportionally. **Applying SPF 50 thinly can give protection closer to a low SPF.**

**UVA is measured separately**, since SPF says nothing about it. UVA penetrates deeper, drives ageing and contributes to skin cancer, and passes through window glass. Look for the **UVA circle logo or a star rating**, and "broad spectrum".

**How to actually use it:**

**Apply generously, before going out**, since chemical filters need time to settle.

**Reapply every two hours**, and immediately after swimming, towelling or heavy sweating — **water resistant is not waterproof.**

**Do not rely on it alone.** Shade, clothing, hats and sunglasses do more, and sunscreen is the last line rather than the first.

**Use it on cloudy days**, since a large proportion of UV penetrates cloud.

**Cover the missed areas** — ears, back of neck, feet, hairline, eyelids.

**Vitamin D** is a genuine consideration in low-sun regions, and supplementation is the answer rather than deliberate burning.

General information, not medical advice.

Question 2

What is eczema, and what actually helps?

Accepted Answer

A condition in which the skin's barrier function is impaired, allowing moisture out and irritants and allergens in, producing inflammation, itching and dryness — and understanding it as a barrier

Question 3

What do clinical trial phases actually mean?

Accepted Answer

Each phase answers a different question, and the questions get progressively harder — which is why most drugs that enter testing never reach patients. Preclinical. Laboratory and animal studies

Question 4

Should you cover a cut or let it breathe?

Accepted Answer

Cover it. The advice to let wounds "breathe" and form a hard scab is widely repeated and contradicted by decades of wound healing research. Why covering works better. The key concept is moist wound

Question 5

What do the two blood pressure numbers actually mean?

Accepted Answer

Blood pressure is written as two figures because the pressure in your arteries changes with every heartbeat. The top number is systolic pressure — the peak, measured when the heart contracts and

Question 6

What changes in a mole should you actually worry about?

Accepted Answer

Change itself is the signal — a mole that is different from how it was, or different from your other moles, matters more than any single feature in isolation. The ABCDE checklist, which is the

Question 7

Why does a bruise change colour as it heals?

Accepted Answer

The colour sequence is your body dismantling the haemoglobin that leaked out of broken blood vessels, and each shade corresponds to a different breakdown product. A bruise forms when blunt impact

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