Sunscreen: what the numbers mean and what they do not

SPF describes UVB. UVA penetrates more deeply, is roughly constant through the day and across the seasons, passes through window glass, and does much of the work of dermal ageing and a good deal of the work in pigmentation. A high SPF with poor UVA filtering is a real product and a bad one.

In Canada, look for broad spectrum on the label, which is a regulated claim. European and Asian products often carry more informative markers: the UVA-in-a-circle symbol, which requires UVA protection of at least a third of the labelled SPF, or the PA+ to PA++++ system. If pigmentation is your concern, prioritise those markers over the SPF number.

Ultraviolet is not the whole of the relevant spectrum. Visible light, particularly the shorter blue-violet wavelengths, induces pigment in deeper skin types and in melasma, and no conventional ultraviolet filter blocks it. Iron oxides do.

This is one of the few areas of skincare where the trial evidence is reasonably clear: in melasma, a tinted sunscreen containing iron oxides outperforms an otherwise equivalent untinted one. If pigment is the concern, the sunscreen should have a tint, and it should be worn indoors near windows as well as outdoors.

Zinc oxide and titanium dioxide are filters. So are avobenzone, octocrylene and the newer organic filters. All of them are chemicals; mineral is shorthand for inorganic. Mineral filters sit better on reactive and post-procedure skin and are less likely to sting the eyes, but they are harder to formulate elegantly on deeper skin tones without a cast. Organic filters tend to be lighter and more wearable, which matters more than it sounds, because a sunscreen that gets worn every day beats a theoretically superior one that does not.

The absorption studies that generated headlines a few years ago showed that some organic filters are absorbed systemically. Absorption is not the same as harm, no clinical harm has been demonstrated, and the regulators have not withdrawn them. If a mineral filter is preferred for comfort or peace of mind, that is a perfectly good reason to use one.

Two-hourly reapplication is an outdoor rule, and it exists because product is removed by sweat, water, towels and hands — not because the filters expire on the skin. A day spent indoors away from windows does not require it. A day at a patio table does.

Powders and sprays are reapplication tools with a quantity problem of their own: almost nobody applies enough of either to reach the labelled protection. They are better than nothing over makeup, and considerably worse than a proper application at the start of the day.

It does not make midday shade unnecessary, and shade, a hat and long sleeves outperform any bottle. It does not reverse existing damage — that is what retinoids, resurfacing and time are for. And it does not make a laser course pointless, though neglecting it will certainly undo one. Pigment treated in the spring and then exposed through a Toronto July comes back, and the person concludes the laser did not work.

Consistency is the whole treatment. A good sunscreen used generously every morning, indoors and out, for a decade, will do more for how skin looks at fifty than anything else available — including everything we offer here.