Shade and timing: the only controls that reduce the dose rather than block it

Everything else on this site is about intercepting ultraviolet after it has arrived. A sunscreen film absorbs or scatters it at the skin surface. A fabric blocks it at the garment. Both are interception, and both can fail: the film can be too thin, the garment can be taken off.

Shade and timing are different in kind. They reduce the amount of ultraviolet that reaches you at all. An hour not spent in the midday sun cannot be undone by a poor application technique, and that makes it the most robust protective decision available.

Timing, in practice

Ultraviolet is driven by sun angle, so it peaks around solar noon and falls away either side. In the UK, public guidance commonly identifies the hours around the middle of the day as the period to avoid, allowing for British Summer Time shifting clock time away from solar time. Our note on the UV index covers the shape of the day.

The point is not that anyone should stay indoors all summer. It is that a great many outdoor activities have more flexibility than people use.

• Training runs and rides can move to early morning or evening, and are usually better then anyway.
• A hill day can start earlier, which also improves the chances of a summit before the weather turns.
• A swim can happen at eight rather than at one.
• Beach time on holiday can split around the middle of the day, which is what people who live in hot countries generally do.
• Garden and DIY work can be scheduled.

What cannot move is a race, a tide, a shift, a swell window or a booked lesson. That is real, and it is why the rest of this site exists. But the fixed cases are fewer than the flexible ones, and people rarely audit which is which.

Shade, in practice

Shade works by removing the direct beam, which is the largest component of ultraviolet exposure. It does not remove diffuse ultraviolet scattered by the sky, and it does not remove ultraviolet reflected from the ground and surroundings. Our note on reflected ultraviolet covers both.

Three practical points about shade.

Deep shade beats dappled shade

A dense canopy or a solid structure blocks far more of the sky than a light tree or an open sided shelter. Dappled shade under a thin tree is a small reduction.

Overhead sun shrinks shade

When the sun is high, an object throws a small shadow directly beneath it. A parasol that shades a whole family at five o'clock in Devon shades one person's shoulders at noon in the Mediterranean. See tropical travel.

Bright surroundings erode it

Under a parasol on white sand, or in the shadow of a sail on bright water, reflected ultraviolet reaches you from the sides and below. That is why people burn in the shade and are surprised.

The hierarchy this publication uses

Every conditions file on this site follows the same order, and it is worth stating it explicitly.

1. Timing. Reduce the dose. No failure mode.
2. Shade. Reduce the direct component. Partial but substantial.
3. Textile. Block completely, for as long as it is worn, with no reapplication. See UPF clothing.
4. Sunscreen. For everything the first three cannot cover, applied generously and reapplied. See reapplication.

Sunscreen is last on that list, and it is not a criticism of sunscreen. It is a recognition that it is the control with the most ways to go wrong: quantity, coverage, timing of application, removal by water, sweat and abrasion, and the absence of any signal when it has gone.

This is not an argument against being outside

Time outdoors is good for people in every dimension that matters, and a publication about sun protection that ended up telling people to stay in would have failed. The argument is about shape, not quantity: the same number of hours outside, arranged differently, produces a very different ultraviolet dose. That is a free improvement and it costs nobody anything except a slightly earlier alarm.

Shade you can carry

Where an activity cannot move in time, it can sometimes move in space, and portable shade is underrated. A tarp at a beach, a shelter at a race finish, a parasol on a boat, an awning at a work site, or simply choosing the shaded side of a street on a walk all reduce the direct component.

Two practical notes. Portable shade works best when it is dense and when it blocks a large part of the sky rather than only the sun. And it works less well on a bright reflective surface, for the reasons above, so a tarp on white sand is a smaller improvement than the same tarp on grass.

Children, prams and the thing not to do

One specific point, because it recurs every summer. Draping a blanket or a muslin over a pram to create shade can significantly reduce air circulation inside it, and public health guidance has repeatedly warned against it in hot weather because of the heat risk it creates. Purpose made pram shades and parasols exist and are designed with ventilation in mind.

More generally, children are the group for whom the timing and shade controls matter most, because they have the least ability to make their own decisions about exposure and the most years ahead in which cumulative exposure accrues. The NHS and Cancer Research UK both publish specific public guidance on children and sun exposure, and it is the right place to read further. We do not give child specific health advice here.

What is not verifiable

We do not publish dose figures for particular hours, shade factors for particular structures, or reductions achieved by particular schedules. We have measured none of it. The mechanism is well established and the practical conclusion does not depend on a number.

The one line version

Move the hours if you can, sit under something solid if you cannot, wear the fabric either way, and let the sunscreen do the small remaining job properly.