Eye protection for curing lights and dental lasers: blue light peaks at 440 nm, the daily limit can be spent in seconds, and laser eyewear has to name a wavelength and an optical density

What it says#

There are two optical hazards in a dental operatory and they behave differently, which is why one pair of glasses does not answer both.

The curing light hazard is photochemical and cumulative. The ADA's dental curing lights topic page states that blue-light retinal injury takes place primarily from exposure in the wavelength range between 380 and 550 nm, with the sensitivity of the retina peaking at approximately 440 nm, and that because camphorquinone absorbs from about 455 to 481 nm with a peak near 469 nm, dental curing units are optimized to perform in exactly the wavelength range where the injury happens. The page reports that under certain clinically relevant conditions the light from dental curing units may exceed dose limit values for photochemical retinal exposure over an 8-hour workday after just under three hours of exposure.

The measured version of that arithmetic is in Labrie and colleagues, published free in the Journal of the Canadian Dental Association in 2011. They cured a simulated incisor restoration from the facial and the palatal with four unit types, plasma arc, low-power LED, high-power LED and quartz-tungsten-halogen, measuring spectral irradiance at 30, 50 and 100 cm, standing in for the operator, the assistant and an onlooker. Weighted against the ACGIH threshold limit values, which in an 8-hour workday are 10 mJ/cm2 for blue light and 3 mJ/cm2 for UV, the maximum permissible cumulative daily exposure to blue light ran from as low as 5.96 seconds, curing from the palatal with the plasma arc unit at 30 cm, to as high as 1.5 hours with the low-power LED from the facial. Put in cycles, an operator 30 cm from a palatally positioned light guide spends the whole day's allowance after two 5-second cycles with that plasma arc unit, two 20-second cycles with the high-power LED, or four 20-second cycles with the low-power LED or the halogen unit. UV was not the problem: the permissible daily UV exposure exceeded 8 hours at every distance and orientation, and the authors concluded the four units tested did not pose a risk of UV-mediated ocular damage.

Distance and loupes change the number. Price and colleagues measured light reflected off a tooth at 40 cm and calculated a permissible cumulative daily exposure of about 11 minutes without loupes; with loupes the irradiance at the pupil was up to eight times greater, but because retinal image area grows with the square of the magnification the effective hazard fell and the permissible time rose to as much as 28 minutes.

The protection is where the evidence turns uncomfortable. Labrie's group notes that ordinary prescription glasses do not prevent penetration of blue or UV light, and that some glasses filtering blue and UV reduce transmission below 500 nm to less than 1 percent; with appropriate eye protection, the same operator could look at the plasma arc unit at 30 cm for a total of 10 minutes a day. Bruzell and colleagues tested 18 protective filters and found nine adequate, those nine transmitting less than 0.1 percent of radiation at any wavelength between 400 and 525 nm. The ADA's own laboratory found 9 of 22 protective filtering devices transmitting blue light from dental curing units at levels ranging from above 4 percent to above 15 percent under clinically relevant conditions. And the ADA states plainly that there is currently no standard that specifically sets test methods, requirements and labeling for protective filtering devices intended for protection against retinal blue light exposure from dental curing units; a working draft sits in a subcommittee of ISO Technical Committee 106, and in the meantime ISO 12609-1 covers eyewear for intense light sources in cosmetic and medical settings with a B-1 to B-6 blue-light classification.

Lasers are a thermal and photochemical hazard governed by wavelength and power, and the eyewear is specified by a number. OSHA's Technical Manual chapter on laser hazards explains the framework: manufacturers classify and label under the federal laser product performance standard at 21 CFR Part 1000, with classes defined at 21 CFR 1040.10 as Class I, IIa, II, IIIa, IIIb and IV; ANSI Z136.1 is the user standard supplying maximum permissible exposures; and optical density is the base-ten logarithm of the worst-case exposure divided by the MPE. Because OD is logarithmic, OD 3 transmits a thousandth of the incident energy and OD 6 a millionth. OSHA reproduces a table of minimum optical densities for direct intrabeam viewing computed from ANSI Z136.1 maximum permissible exposures, which includes a 1 W argon at 0.514 um needing OD 3.4 for 10 seconds, 5.2 for 600 seconds and 6.4 for 8 hours; a 5 mW helium neon at 0.633 um at 1.1, 1.7 and 2.9; a 50 mW gallium arsenide diode at 0.840 um at 1.8, 2.3 and 3.7; a 100 W Nd:YAG at 1.064 um at 4.7, 5.2 and 5.2; and a 1000 W CO2 at 10.6 um at 6.2, 8.0 and 9.7. The same chapter notes that eye protection is generally required for Class IIIb and Class IV lasers and that all laser eyewear shall be clearly labeled with OD values and the wavelengths it protects against.

What is not there is a federal rule with a number in it. OSHA's chapter says that at present OSHA does not have a comprehensive laser standard, that the construction rules at 29 CFR 1926.54 and the personal protective equipment subpart may apply, and that citations are issued by invoking the general duty clause or Subpart I, requiring employers to work to industry consensus standards such as ANSI Z136.1. The construction eye-protection rule, 29 CFR 1926.102(b)(2), is the one that spells out the requirement in regulatory language: employees exposed to laser beams shall be furnished suitable laser safety goggles which will protect for the specific wavelength of the laser and be of optical density adequate for the energy involved. For general industry, which is where a dental office sits, the enforceable hook is 29 CFR 1910.133(a)(1): the employer shall ensure that each affected employee uses appropriate eye or face protection when exposed to eye or face hazards from, among other things, potentially injurious light radiation.

Who it applies to#

Everyone in the room. The curing light exposure times above were calculated at 30 cm for an operator, 50 cm for an assistant and 100 cm for an onlooker, and the assistant's numbers are only a few multiples better than the operator's. The ADA's Eye Safety in the Dental Office page adds the patient: protective eyewear goes on as the patient sits down and comes off as they leave, patients with their own prescription glasses still need it, and contact lenses provide no barrier. It also notes that specific protective eyewear is designed for procedures using lasers, light curing units and bleaching units, and that each manufacturer's product literature governs which is which.

Two limits on this entry are worth saying out loud. First, ANSI Z136.1 and the health care companion Z136.3 are sold documents; nothing in them is quoted here beyond the way OSHA characterizes them in its own free manual. Second, OSHA's optical density table was computed for the laser types in the table, and it does not include the erbium wavelengths near 2,780 and 2,940 nm that some dental lasers use. For those, the number comes from the device manufacturer and the eyewear supplier, on the label, not from anything free that could be verified here.

What changed#

The lights got stronger and the guidance did not keep up. The quartz-tungsten-halogen units studied in the 1980s delivered around 300 mW/cm2 and were judged to have little potential to cause ocular injury; the plasma arc and LED units in the 2011 study deliver up to 3,000 mW/cm2 or more, with narrow spectra centered near the retina's most sensitive wavelength. LED units are also less likely to trigger the eye's aversion response, which normally limits an unintended glance to about a quarter of a second.

Meanwhile the regulatory picture stayed empty. Fluent and colleagues wrote in JADA in 2019 that the CDC's dental infection control guidelines and OSHA's bloodborne pathogens standard include no recommendations or regulations directly related to blue light exposure, that other OSHA regulations do require employers to protect employees from potentially injurious light radiation, and that it is not readily evident to employers or dental personnel that those regulations apply. The ADA's 2023 page confirms the gap is still open.

Where the source is#

The ADA's Dental Curing Lights topic page, last updated March 27, 2023, is the link under the title and is the best single free summary of the blue light hazard, the filter testing and the standards gap. Labrie and colleagues' 2011 paper is free in full text at the Journal of the Canadian Dental Association and contains the measured exposure times and the cycle counts. The Bruzell filter study and the Price loupes study are behind publisher access; their abstracts, which are public, carry the figures quoted here. OSHA's Technical Manual, Section III Chapter 6, is free and carries the laser classes, the optical density definition and table, and OSHA's own statement that it has no comprehensive laser standard. The federal laser product classes are at 21 CFR 1040.10 on eCFR, and the general-industry eye protection rule is 29 CFR 1910.133. ANSI Z136.1 and Z136.3 are sold by the Laser Institute and are not quoted here.