An electric arc produces intense electromagnetic radiation spanning the full optical spectrum, from deep ultraviolet through visible light to infrared, along with significant thermal energy. This radiation is the primary reason welding arcs are dangerous to look at and why arc flash events in electrical systems cause severe burns. The physics, the health consequences, and the protection strategies are all shaped by what wavelengths the arc emits and how intensely it emits them, which varies considerably depending on the current, the gas environment, and the metals involved.
What an Electric Arc Actually Emits
An electric arc is a sustained electrical discharge through a gas, and the extreme temperatures involved (often exceeding 10,000 K at the core) cause the gas and any vaporized material to emit radiation across a wide range of wavelengths. The principal radiation in the ultraviolet and visible bands comes from electronic transitions in atoms, ions, and molecules, while infrared emission arises mainly from vibrational and rotational transitions in diatomic and triatomic molecules, along with recombination of ions and electrons.
1Plasma Sources Science and Technology. Foundations of plasma photonics: lamps, lasers, and electromagnetic devicesIn practical terms, the arc acts like a small, extremely bright light source that radiates across the ultraviolet (UV), visible, and infrared (IR) bands simultaneously. Welding arc measurements typically cover wavelengths from about 200 nanometers (deep UV) to 1,000 nanometers and beyond into the thermal infrared.2Industrial Health. Spectra of Optical Radiation from Welding Arcs The relative intensity of each band depends heavily on what is happening inside and around the arc, which is why two different welding setups or two different electrical faults can look and feel quite different to a bystander.
How Metal Vapor and Gas Composition Shape the Spectrum
One of the less intuitive aspects of arc radiation is that the surrounding atmosphere and the materials being consumed by the arc dramatically change what comes out. Research on long, high-power arcs (up to two meters long, carrying currents between 4,000 and 40,000 amps) using copper, steel, and aluminum contacts found that the metal vapor had a large influence on visible and UVA radiation, while the infrared output was driven primarily by the air plasma and the hot gases and fumes surrounding the arc.3Journal of Physics D: Applied Physics. Radiation of long and high power arcs This means the material the arc is burning through partly determines which wavelengths dominate. An arc vaporizing aluminum produces a different spectral signature than one consuming steel, even at the same current.
The shielding gas used in welding processes also matters. In gas metal arc welding of mild steel, switching from pure carbon dioxide shielding to an argon-COâ‚‚ mix changed the UV output substantially. At lower welding currents (below about 250 amps), the two gas mixtures produced roughly equal UV irradiance. But above 300 amps, the argon-rich mixture produced significantly more UV than pure COâ‚‚.4PubMed Central. Hazard of ultraviolet radiation emitted in gas metal arc welding of mild steel Pulsed current modes also drove UV levels much higher compared to non-pulsed welding at the same average current, a finding that matters for anyone choosing welding parameters or assessing exposure risk.
The comparison extends to the type of power supply. The spectra of arcs generated under alternating current and direct current have been compared across the UV, visible, and near-infrared range, and the differences are measurable, though the overall spectral shape (UV through IR) is present under both conditions.5Energies. Comparative Analysis of Optical Radiation Emitted by Electric Arc Generated at AC and DC Voltage The practical takeaway is that you cannot assume all arcs are equally hazardous. The specific process, current level, material, and gas environment all shift the spectral output, sometimes dramatically.
Eye Injuries from Arc Radiation
The most familiar hazard of arc radiation is to the eyes. The intense UV output of an arc can cause photokeratitis, sometimes called “arc eye” or “welder’s flash,” which feels like sand in the eyes accompanied by pain, tearing, and light sensitivity.6International Journal of Health Sciences and Research. Assessment of Effective Irradiance from UV Radiation in Some Welding Workshops within Ogbomoso, Nigeria This is essentially a sunburn on the cornea. It usually heals within a day or two, but the experience is unpleasant enough that most people who have had it once become serious about eye protection afterward.
More concerning is the potential for permanent retinal damage. A documented case involved a 47-year-old man who performed arc welding for only about 10 to 15 minutes without eye protection because he was working in a tight space. He subsequently developed decreased visual acuity in both eyes, with imaging showing disruption of the retinal layer responsible for light detection and reduced electrical responses in the central part of both retinas. His reduced vision persisted for at least 18 months.7PubMed Central. Maculopathy from an accidental exposure to welding arc This is not photokeratitis, which affects the front of the eye and heals. Maculopathy from arc exposure affects the retina itself, and recovery is uncertain. The fact that it took only minutes of unprotected exposure to produce lasting damage underscores how intense the radiation from an arc really is.
The UV component gets the most attention in eye hazard discussions, but visible light and blue light from arcs are also relevant, particularly for bystanders who may not be looking directly at the arc but are still exposed to its glare. Assessment of plasma arc cutting operations found that the intensities of visible light, UV-C, and UV-B often required specific shade lens numbers for safe viewing.8PubMed. Evaluating optical hazards from plasma arc cutting Even processes that are not traditional welding can produce optical radiation intense enough to require proper eye protection.
Skin Effects and Long-Term Cancer Risk
The skin hazard from arc radiation is often underestimated compared to the eye hazard, partly because the immediate symptom, erythema (a sunburn-like reddening), seems minor and heals quickly. But welders are exposed to the full spectrum of UV radiation from the arc, and they frequently experience localized burns and skin reddening on exposed areas.9PubMed Central. Skin cancer and welding UV from gas tungsten arc welding of aluminum alloys, for example, is known to frequently cause both keratoconjunctivitis (eye inflammation) and erythema.10Industrial Health. Hazard of ultraviolet radiation emitted in gas tungsten arc welding of aluminum alloys
The longer-term worry is skin cancer. A study following welders over time found a significantly higher risk of basal cell carcinoma among those exposed to welding for more than 30 years, with roughly two and a half times the risk compared to unexposed individuals. The same study found that diagnoses of actinic keratosis (a precancerous skin lesion) on the neck were about two and a half times more common among welders with more than 20 years of exposure.11PubMed Central. Metal arc welding and the risk of skin cancer The neck is telling because it is one of the areas most frequently left uncovered by standard welding helmets and clothing, catching UV that scatters or reflects from the workpiece and surroundings.
These findings are consistent with what you would expect from chronic UV exposure, similar to the pattern seen in outdoor workers with cumulative sun damage. The difference is that welding arcs can produce UV intensities far greater than sunlight over the small area directly exposed, which means the dose accumulates faster per unit of exposed skin. The cancer risk is concentrated in workers with decades of exposure, but the acute burns start from the first unprotected session.
Ozone and Other Secondary Effects
Arc radiation does not just affect people directly. The UV output from a welding arc irradiates the surrounding air and generates ozone, a respiratory irritant. The amount of ozone produced depends on the welding process, the material, and the current. Under natural ventilation conditions, ozone levels near the welder’s breathing zone ranged from 0.06 parts per million for flux-covered manual electrodes up to 0.47 ppm for bare-wire argon-shielded welding of aluminum.12Oxford Academic. Ozone in Arc Welding The occupational threshold limit has traditionally been set at 0.1 ppm, meaning certain welding configurations can exceed it by nearly five times without mechanical ventilation.
The ozone problem is largely solvable with proper exhaust ventilation, which can bring levels below the occupational limit. But it is another example of how arc radiation creates hazards beyond the obvious light and heat. If you are welding in an enclosed or poorly ventilated space, the ozone and other fumes generated by UV interacting with the atmosphere are a genuine respiratory concern separate from the metal fumes that most welders already know about.
Arc Flash in Electrical Systems
Outside of welding, the other major context where arc radiation matters is the arc flash, an unintended electrical arc that occurs during faults in power distribution equipment. An arc flash releases an enormous burst of energy in a fraction of a second, and the thermal radiation component is a major cause of serious burn injuries to electricians and maintenance workers.13ScienceDirect. Electric arc explosions—A review
The physics of the radiation is the same as in a welding arc, but the practical experience is very different. A welding arc is controlled, sustained, and expected. An arc flash is sudden, often explosive, and accompanied by a pressure blast and molten metal spray. The thermal radiation from an arc flash can ignite clothing and cause severe burns at distances of several feet from the fault. This is why arc-rated clothing and strict safety protocols around live electrical panels exist. The radiation intensity from a high-energy arc flash can be orders of magnitude greater than from a typical welding arc because the available fault current in industrial electrical systems can be enormous, sometimes tens of thousands of amps with no deliberate current-limiting as there is in a welding machine.
The design of protective clothing for arc flash scenarios has received considerable research attention, and the standards are different from welding protection because the exposure is a single brief pulse rather than a sustained source. Arc-rated garments are tested for their ability to resist ignition and limit heat transfer during a short, intense thermal event, rather than for their ability to block UV over an eight-hour shift.
How Protective Clothing and Screens Work Against Arc Radiation
For welding, where the exposure is sustained and predominantly UV, the protective approach centers on blocking ultraviolet transmission through fabric and through welding screens that shield bystanders. Testing of various workwear fabrics against actual welding arc spectra found that the heaviest fabrics (around 350 grams per square meter) were essentially opaque to UV, with transmittance so low it was negligible. Lighter fabrics allowed slightly more UV through, and dark-colored fabrics consistently outperformed white ones in UV blocking.14PubMed Central. Photoprotection by Workwear: Ultraviolet Protection Factors for Artificial Radiation from Welding Arcs This is analogous to what you might expect from clothing for sun protection, but tested against the actual spectral output of a welding arc rather than sunlight, which matters because the relative UV intensity is different.
For bystanders and co-workers in shared spaces, welding screens and curtains serve a similar function. International standards set maximum transmittance values that screens must meet for infrared, ultraviolet, and blue light at a minimum distance of one meter from the arc, ensuring that anyone on the other side of the screen is not exposed above accepted limits.15PubMed. Welding screens and curtains: Part 1-the derivation of transmittance requirements The green or amber tinted curtains you see in welding shops are not decorative; they are specifically designed to absorb the UV and blue-light wavelengths that are most hazardous while still allowing enough visible light through for situational awareness.
The shade number on a welding lens is the most familiar form of arc radiation protection. Different welding processes and currents call for different shade numbers because the intensity of the visible and UV output varies. A higher shade number means less light gets through. The shade requirements published by occupational safety agencies are derived from the spectral characteristics of various arc types and the exposure limits for eyes and skin. For plasma arc cutting, measured intensities sometimes fall below the shade numbers stipulated in standard tables, suggesting that the published guidelines build in a safety margin.16PubMed. Evaluating optical hazards from plasma arc cutting That margin exists because real-world conditions are variable: the arc might flare, the welder might shift position, or the process parameters might drift.
Why Current Level Matters So Much
If there is a single variable that most strongly predicts how much radiation an arc produces, it is the current. Higher current means more energy passing through the arc, which means a hotter, brighter plasma and more radiation at every wavelength. In gas metal arc welding of mild steel, UV irradiance increased with current across every shielding gas tested. The range was dramatic: using a non-pulsed argon-COâ‚‚ mix, effective UV irradiance went from about 0.5 milliwatts per square centimeter at 100 amps up to roughly 13 milliwatts per square centimeter at 350 amps.17PubMed Central. Hazard of ultraviolet radiation emitted in gas metal arc welding of mild steel That is a roughly 25-fold increase over a current range that many welding shops routinely span.
The jump was not linear, either. The increase accelerated above about 250 amps, meaning that going from 250 to 350 amps increased the UV hazard disproportionately compared to going from 100 to 200 amps. This has real implications for anyone setting up a welding operation: a modest increase in current at the high end of the range can require a meaningful upgrade in protective measures. Pulsed current modes amplified the effect even further, producing UV levels at moderate average currents that matched or exceeded non-pulsed welding at much higher currents.
For arc flash in electrical systems, the same principle applies at a larger scale. Fault currents of 10,000 amps or more produce correspondingly intense radiation. The available fault current at a given point in an electrical system is one of the key inputs to arc flash hazard calculations, which determine what level of protective clothing workers must wear when working near energized equipment.
Measuring Arc Radiation in Practice
Quantifying the radiation from an arc is not as simple as pointing a light meter at it. The measurement must be spectrally resolved, meaning you need to know how much radiation is coming out at each wavelength, not just the total. This is because different wavelengths pose different hazards: UV-C and UV-B are most damaging to the cornea, UV-A penetrates deeper into the eye and skin, visible light and blue light can cause retinal damage, and infrared contributes to thermal burns and cataracts over time.
Standard measurement approaches involve spectroradiometers positioned at a known distance from the arc (typically one meter for standardized comparisons), with the irradiance then calculated using the inverse-square law for other distances. In research on long, high-power arcs, measurements were taken at 10 meters from the arc axis and integrated across four spectral bands corresponding to UV, visible, near-infrared, and far-infrared.18Journal of Physics D: Applied Physics. Radiation of long and high power arcs The variability between measurements can be substantial because arcs are inherently unstable, flickering and shifting in position and temperature from moment to moment. Multiple measurement sets are typically averaged, and the spread between readings can be large.
For practical hazard assessment, the raw spectral data is weighted by biological effectiveness functions that account for how sensitive the eye or skin is at each wavelength. A milliwatt of UV-C is far more damaging to the cornea than the same power in visible light, so the weighting factors for short-wavelength UV are much higher. The resulting “effective irradiance” number is what gets compared against occupational exposure limits to determine how long someone can safely be exposed and what protection they need.
Bystander Exposure and Distance
A common misconception is that arc radiation is only a concern for the person operating the equipment. In reality, anyone within line of sight of the arc is exposed, and the UV component can cause eye and skin effects at surprisingly large distances. Because irradiance falls off with the square of the distance, moving from one meter to three meters away reduces the exposure by about a factor of nine. But the initial intensity is so high that even at several meters, unprotected eyes can receive enough UV to cause photokeratitis within minutes.
This is the reason welding screens and curtains are required in shared workspaces, and why the reassessment of screen transmittance requirements considers hazards at distances of at least one meter.19PubMed. Welding screens and curtains: Part 1-the derivation of transmittance requirements In open fabrication shops, it is not unusual for workers across the room to experience mild arc eye symptoms if they repeatedly glance toward an unscreened welding operation. Reflective surfaces (polished metal, white walls, water) can also redirect arc radiation into areas that seem shielded from the direct arc.
For arc flash events in electrical systems, the bystander hazard is thermal rather than primarily UV, but the principle is the same. The thermal radiation from a high-energy arc flash can cause second-degree burns through clothing at distances that surprise people who have not worked around energized equipment. Safety engineers calculate “arc flash boundaries,” the distance at which the incident energy drops below the threshold for second-degree burns on bare skin, and these boundaries can extend several feet from the equipment depending on the available fault current and the clearing time of the protective devices.
Processes That Produce Unexpectedly High UV
Not all welding processes are equal in their UV output, and some that seem routine produce surprisingly intense ultraviolet radiation. Gas tungsten arc welding (TIG) of aluminum alloys is a well-known high-UV process, frequently causing both eye inflammation and skin erythema among welders.20Industrial Health. Hazard of ultraviolet radiation emitted in gas tungsten arc welding of aluminum alloys The reasons include the argon shielding gas (which is more transparent to UV than COâ‚‚), the reflective surface of aluminum (which bounces UV back toward the welder), and the relatively clean arc that lacks the UV-absorbing fume cloud produced by flux-coated electrodes.
Manual stick welding with flux-covered electrodes, by contrast, tends to produce the least UV because the flux creates a dense fume cloud and slag that absorb short-wavelength radiation before it escapes. This is consistent with the ozone data: flux-covered electrodes produced the lowest ozone levels (around 0.06 ppm), while bare-wire argon-shielded aluminum welding produced the highest (0.47 ppm), reflecting the much greater UV output that drives ozone formation.21Oxford Academic. Ozone in Arc Welding
Plasma arc cutting is another process worth noting. Although it is not welding in the traditional sense, the arc in a plasma cutter is extremely hot and concentrated, producing significant optical radiation. Measurements have shown that the UV-C and UV-B components require specific lens shade protection, though in some cases the measured levels were below what standard safety tables would suggest for a given process type.22PubMed. Evaluating optical hazards from plasma arc cutting The variability between processes and setups reinforces the point that generic assumptions about “welding radiation” can underestimate or overestimate the actual hazard, depending on the specific situation.