At two in the morning a middle-aged man walked into the emergency department with both hands pressed over his eyes. He had been fine all day. He finished his shift, went home, took a shower, and then, out of nowhere, both eyes felt as though they were full of sand — burning, streaming, unable to face the light. The doctor asked what he did for a living. Welder, and he had been pushing a deadline all day. Diagnosis: photokeratitis, arc eye.

On site, welders call it "getting flashed". The most confusing part is this: nothing happens while the work is being done — it flares up hours after the shift ends. Many people therefore never put it down to the arc they looked into that afternoon. They turn up the next day, work as usual, and take another dose.

Why arc light settles its account late

The mechanism is not complicated. Ultraviolet radiation from the arc is absorbed in quantity by the epithelial cells of the cornea and conjunctiva; the cell proteins denature, the nerve endings are left exposed, and the pain arrives. The latency is well defined — typically 6 to 8 hours after exposure, as short as about half an hour, and no longer than 24 hours — which is exactly why it prefers to ambush people in the middle of the night. A health explainer from People's Daily sets out the timing plainly: onset at night is typical, and that is what makes the condition so easy to dismiss.

Repeated attacks are not something a night's sleep settles either. Multiple episodes of photokeratitis can develop into chronic conjunctivitis and corneal degeneration, with vision drifting downward. For a welder this is not one uncomfortable night to be endured; it is a real entry on the occupational health ledger.

If it does happen, remember three steps: leave the ultraviolet environment immediately; rest with the eyes closed, and a cool wet towel eases the pain; and seek medical attention promptly so the corneal epithelium is properly treated. Never rub hard with your hands — the epithelium is already softened and lifting, and rubbing only enlarges the damaged area.

Shade number: the hard currency of arc protection

What blocks arc light? The shade number of the welding filter. Under GB 3609.1—2025, Eye and face protection — Welding protection — Part 1: Welding protectors, the shade number is the designation for the darkness of a filter, or the degree to which luminous transmittance is attenuated — the higher the number, the darker the lens and the less light that passes through.

Here is a date many crews still have not registered: the old welding eye and face protection standard, GB/T 3609.1—2008, has been replaced by GB 3609.1—2025. Note that the new number carries no "/T" — it is a mandatory national standard, issued on 29 August 2025 and in force from 1 September this year. Anyone specifying welding protection now goes by the new number.

Incidentally, the new standard is split into two parts. Part 1 covers welding protectors with a fixed shade number — welding spectacles, welding goggles, welding helmets and black glass welding filters — while automatic welding filters are provided for separately in Part 2. Each part sets out its own technical requirements and test methods in full.

The part of the new standard most relevant to the shop floor is the shade selection guidance in Annex G. Its reference conditions are a distance of about 50 cm from the eyes to the weld pool and an ambient illuminance of 100 lux. For the most common process — manual metal arc welding with covered electrodes — the range given by current is shade 8 to 14, a higher current calling for a higher number; TIG welding falls between 8 and 13; gas welding is selected by acetylene flow from 4 to 6, and oxygen cutting by oxygen flow from 5 to 7. For the exact number at any given current level, follow the comparison table in Annex G.

Two provisions that are frequently overlooked are worth copying onto the crew room wall:

One concerns welding assistants. The standard states that helpers, like anyone else inside the welding area, must also be protected, and products of shade 1.2 to 4 may be selected; it adds a pointed note — where a helper stands as close to the arc as the welder does, both should use the same shade number.

Two concerns passers-by. People who are neither welders nor assistants, but simply cross a welding area or pause nearby, should also be given eye and face protection. Arc light does not choose its targets. A sideways glance still delivers a real dose.

In addition, the standard sets hard requirements for the light transmission of the non-filter area of a helmet and for the transmittance of the cover plate (cover plate luminous transmittance not less than 80%). Products claimed to protect against high-speed particles must also meet the relevant impact class and provide side protection — spatter and arc light, covered together.

Pitfall 1 — treating "I can't see it clearly" as "I'm protected enough"

The two opposite mistakes seen most often on site:

One is choosing too dark a shade. Some people assume darker is safer, then cannot see the weld pool, lift the helmet a crack after two passes, or tilt their head to squint through the gap — the arc goes straight into the eye and protection drops to zero. The logic of shade selection is not "as dark as possible" but "dark enough while the weld pool stays readable"; the standard also states that the number may be raised or lowered by one step according to the operation or the environment.

The other is substituting the wrong eyewear. Ordinary sunglasses or clear impact goggles on a welding post let ultraviolet straight through — a clear lens stops spatter, but it does not stop arc light. The reverse mismatch is just as wrong: using a black glass welding filter at a grinding post leaves you dark, unable to see and badly matched to the job.

There is a quieter one too. Helpers and the workers grinding alongside them spend years working at the edge of the arc with bare eyes, and assume there is no problem because it has never flared up. The latency mechanism of photokeratitis means whether it flares up has nothing to do with whether your eyes hurt that day. The damage accumulates.

Four things you can do this week

1. Take stock. Check every welding filter in use in the workshop against its process and current — manual metal arc welding within the 8 to 14 range — and fill the gaps in the shade numbers you hold.

2. Read the marking. For new purchases, look for GB 3609.1—2025, and make sure the relation between the shade number and luminous transmittance is clear and verifiable on the packaging. If you cannot read it, it is better not to buy it.

3. Issue eye protection to assistants. Keep goggles of shade 1.2 to 4 available at the point of issue; where a helper works as close to the arc as the welder, issue the same shade number.

4. Brief the night shift. Red eyes, a gritty feeling, light sensitivity and streaming tears a few hours after the shift ends: do not rub, get medical attention promptly. That is photokeratitis, and it must not be allowed to become a recurring injury.

Photokeratitis is one of the few welding injuries that settles its account hours late, which is precisely why the shade number on the filter — and the discipline to keep it there — matters more than any argument about whether the arc looked bright. For procurement teams sourcing welding protection and safety managers writing site rules, the current mandatory standard is the baseline to specify against. CK Tech supplies welding helmets, welding filters and eye and face protection with shade marking, luminous transmittance and impact performance documented for verification.

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This article is general safety information. In practice, follow the current text of GB 3609.1—2025 and your own site safety management rules.