People glance at the day’s peak, see Moderate, and treat the afternoon as open season. Or they duck a 20-minute errand at Very High, then garden for three hours at 4.

The index is how fast UV arrives. Time writes how much you get. Stay out long enough at a “mild” number and you can outdose a short spike.

Index times hours

WHO’s Global Solar UV Index is 40 times the erythemally weighted irradiance in watts per square metre. The CIE Standard Erythemal Dose (SED) is 100 J/m² of that same weighted UV. Put the two together and the rate is blunt:

About 0.9 SED per hour for each index point.

Occupational tables that use that conversion list:

• UV 4 — 3.6 SED/hour
• UV 8 — 7.2 SED/hour
• UV 12 — 10.8 SED/hour

Hold those rates still for a worked example—not a forecast for your lawn:

• Three hours at a steady 4 → 10.8 SED
• Twenty minutes at a steady 8 → 2.4 SED

The “easier” outing is more than four times the dose.

ICNIRP’s indicative unadapted MED—the dose for a just-perceptible reddening—is about 2 SED for phototypes I–II. So 2.4 SED is roughly one burn unit for fair, unadapted skin. 10.8 SED is several of those units.

The ACGIH/ICNIRP occupational skin goal, translated onto the CIE scale, sits near 1.0–1.3 SED in an eight-hour day. At a steady UV 4, that budget is gone in about 20 minutes of open sky. At UV 8, closer to 10 minutes. The rest of a long Moderate block is extra.

The trap is reading only the peak

Three everyday misses:

1. “It’s only a 4.” WHO already wants cover at 3 and above. Moderate is a slower tap, not an empty one.
2. “I avoided midday.” Moving a three-hour hike into a Moderate morning can still beat a 15-minute noon errand. UV peaks before the heat is about when the spike lands. This is about how long the meter runs.
3. “Sunscreen lets me stay.” WHO’s practical guide is blunt: sunscreen should never be used to prolong time in the sun. Clothing and shade first; sunscreen is for what they miss.

A towel on the grass is closer to the full horizontal dose. A standing gardener or cyclist gets less on vertical skin. ICNIRP’s outdoor-worker note is useful here: on a very high ambient day (~40 SED), uncovered arms and legs might still take about 10 SED, shoulders 20–30. Posture cuts the absolute number. It does not flip the comparison. Same body, same sky geometry: longer at 4 still outruns shorter at 8.

Plan the length, then the peak

Use the live UV index for the rate. Then name how long you will actually be out:

1. Read today’s peak and the next few hours—then the block: 20 minutes, two hours, all afternoon.
2. If the block is long and the index sits at 3 or above, treat it as a cover day even when the label is only Moderate.
3. Split the time. Shade, an indoor break, a shirt. Do not use sunscreen as a permit to add hours.
4. Do not trade a short Extreme spike for an unprotected marathon at 4. Both need cover; the marathon needs it for longer.
5. Recheck. Clouds and the dropping afternoon sun change the rate. The hours you already spent still count.

A short shadow still flags a high rate—see a shadow shorter than you. That check answers “is it strong right now.” It does not answer “how much have I banked since breakfast.”

Sources

The UV index is a rate: WHO defines UVI with k_er = 40 m²/W in the UV index Q&A. SED arithmetic uses the CIE standard erythemal dose (100 J/m²). Occupational tables (including ARPANSA) have used ~10.8 SED/hour at UV 12 for a flat, unshaded surface. Real skin, shade, and a changing sky move personal dose. ICNIRP MED ranges are not a personal timer.