Sunglasses: Categories 0 to 4, and Why Not 4 at the Wheel

A snowfield in full sun, just past noon, not a cloud in sight. You squint, then dig out your everyday sunglasses: category 2, nice frames, bluish lenses. They look great. They’re just not built for this. It all comes down to two lines the label keeps carefully apart: the category, which says how much light gets through, and the UV marking, which says how much of the invisible part you let in.

What does the category, from 0 to 4, really measure?

The ISO 12312-1 standard sorts sunglasses lenses into five categories, based on the share of visible light they let through, known as luminous transmittanceThe share of visible light that passes through the lens, as a percentage. The lower it is, the darker the lens. [1]. The darker the lens, the higher the category and the lower the transmittance. So the category is about comfort and glare. On its own, it says nothing about protection from UV.

The step from category 3 to category 4 is bigger than it looks: the light transmitted drops from 8 to 18% down to 3 to 8%, less than half as much. That’s what makes category 4 great on a glacier, and unusable as soon as your surroundings get dark.

Does a dark tint protect better from UV?

No, and it’s the most common mix-up. The Swiss office puts it plainly: UV protection depends on the absorbing properties of the lens material, not on its color or tint [2]. The U.S. FDA adds that clear, green, amber, red or gray lenses can protect as well as very dark ones [3].

What to look for is the “100% UV” or “UV400” marking and, on pairs sold in Europe, the CE mark. For the FDA, the UV400 marking corresponds to more than 99% of UVA and UVB blocked [3]. The standard defines that 100% as under 0.05% transmission at wavelengths of 380 nm and below [2]. The Bundesamt für Strahlenschutz, Germany’s Federal Office for Radiation Protection, notes that measurements show the UV400 marking can be trusted fairly well [4]. The French consumer organization INC states that the CE mark must appear on an arm of the glasses for them to be sold in France and the European Union [5].

Two pieces of good news and one less welcome. In a test commissioned by the Swiss office, all 57 pairs bearing the CE mark met the requirement of 100% UV protection, and no difference in optical quality was seen between expensive and cheap pairs [2]. The less welcome one: protection ages. The standard’s wear test aims at safe use for two years, at two hours of wear a day, and scratched lenses protect less [2].

Why snow and altitude change everything

Two effects add up. First, altitude: there’s less atmosphere above you to filter the UV. Assurance maladie, France’s national health insurance, counts a rise of 10 to 12% per 3,300 ft (1,000 m) [6], while the UIAA, the international mountaineering federation, uses 4% per roughly 1,000 ft (300 m) of ascent [7], which comes to about 13% per 3,300 ft (1,000 m).

Then there’s the snow, which sends back up what the sky sends down: about 80% of UV according to the UIAA [7] and Assurance maladie [6], 85% according to the UK’s Met Office [8], and up to 90% for fresh snow according to Germany’s radiation protection office [9]. The sources don’t agree to within a few points, so just remember the order of magnitude: most of the UV comes back toward your face. The Met Office even estimates that your eyes can get twice as much UV during winter sports [8].

The result is called snow blindness, or photokeratitis: a sunburn of the cornea [6]. The trap is the delay. According to the UIAA, the damage is done before the symptoms show up: a few minutes without protection on a high-altitude glacier can be enough, and the signs sometimes arrive only the next night [7]. The American Academy of Ophthalmology describes pain, blurred vision, swelling and watering eyes [10].

The sides matter as much as the lenses

Without side protection, your eye takes in the UV reflected by its surroundings. The Swiss office considers side protection essential for category 4 and recommended for all the others, to limit pterygium and cataract [2]. The German office prefers glacier glasses with side protection for winter sports [9], and the UIAA speaks of glasses with side shields or goggles [7]. Even so, glasses don’t block everything: a direct ray can sneak around them, and another comes from the ground or the diffuse sky [2]. For skiing and snowboarding, the American Academy of Ophthalmology recommends goggles with polycarbonate lenses, which generally give better peripheral vision than wraparound glasses [10]. Even snowshoeing as a beginner on marked trails is no reason to skip this protection.

Category 4: perfect on the glacier, out of the question at the wheel

The trailhead parking lot is where category 4 becomes a problem. According to the UK’s College of Optometrists, which summarizes the standard, the filters suited to the road are categories 0, 1, 2 or 3, not 4 [1]. The standard also asks that traffic lights stay easy to recognize [1]. The Swiss office explains that these lenses are too dark to tell traffic lights apart [2]. The French consumer organization INC adds that they prevent proper perception of depth, color and contrast [5].

The wording changes from one country to the next, but the conclusion doesn’t. In Switzerland, the office calls such lenses “not permitted in road traffic” [2], as does the German office [4]. In France, the Sécurité routière, the French road safety agency, writes that category 4 lenses, like those of ski glasses, are “strongly discouraged” for driving [11], and the INC says not to wear them at the wheel [5]. A pair that isn’t suited to the road also carries a crossed-out car pictogram according to the Swiss office [2], or a warning in France according to the INC [5].

When the light fades, no sunglasses will do. A filter that transmits less than 75% of the light isn’t suited to the road at dusk or at night [1], ADAC, the German automobile club, advises taking sunglasses off as soon as daylight goes [12], and the Swiss office advises against wearing them at night, at dusk or in poor light [2]. My take: a second pair, category 2 or 3, in the glove box takes care of the drive down.

Polarized, photochromic, lens color: are they worth it?

A polarized lens filters reflected light, and so the glare from flat surfaces such as water, wet roads or windows [5]. For snow, the UK’s Met Office [8] and the U.S. National Eye Institute [13] find it useful. On the road, opinions differ: ADAC reports the view of an ophthalmologist who says polarization only works under very specific laboratory conditions, not in everyday driving [12]. Either way, polarization and UV protection are two different properties: check the second one on the label.

Photochromic lenses darken under the effect of UV [5]. Behind the wheel, that’s their flaw: the windshield stops a good share of the UV they need, their reaction time of 30 seconds or more is judged unacceptable at a tunnel entrance, and even when clear they still absorb 20 to 30% of the light, according to ADAC [12].

That leaves color. Brown and gray distort colors the least, and so traffic lights too; blue and red hurt contrast and sharpness, according to the German and Swiss offices [4] [2]. ADAC judges yellow-filter lenses permanently unsuitable for the road [12].

Which pair for which outing?

What to check before you head out

1. Read the label: UV400 or 100% UV (and the CE mark on European pairs), category, and any note on driving.
2. For snow at altitude, aim for category 3 or 4 with side protection, or goggles.
3. Pack a second pair, category 2 or 3, for the road and as a spare.
4. Replace scratched lenses and pairs more than two years old.
5. Put them on in the first few minutes on snow, even when the sky is overcast.

One more layer of protection: near a reflective surface such as snow or a glacier, the Ligue contre le cancer, a French cancer nonprofit, recommends UV-protective clothing [14]. And the smart move fits in one sentence: the category sets the light, the UV marking protects the rest, and category 4 stays on the glacier.

Guillaume Hoarau

Camping, backpacking, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, backpacking, bushcraft, and hiking. A survival-skills buff and a collector of vintage field manuals, he loves clear explanations, lists that actually help and other people’s stories, which he tells well.

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