Ultraviolet safety assessments of insect light traps

Instrumentation

Initial field measurements of a variety of UV-A ILT lamp fixtures were made using a broadband radiometer, an International Light Model 1400A radiometer/ photometer (S.N. 1402). Two detectors were used: a Model SEL240 detector with input optic T2ACT3 calibrated to read directly in terms of the ACGIH/ICNIRP UV-Hazard S(λ)-weighted effective irradiance; [10] and a Model SEL033 UV-A detector (with input optic and UVA filter) calibrated to measure near-ultraviolet (UV-A) radiation between approximately 315 and 400 nm. However, the first (UV safety) detector appeared to be responding too strongly to the UV-A and provided high readings. Therefore, an attempt was then made to use a small, portable spectroradiometer to check these earlier readings: a calibrated Ocean Optics Spectroradiometer (Model USB2000 with a 180°-cosine-corrected irradiance probe). Although this instrument permitted cross-comparisons with the more portable IL-1400A hand-held meter, it also appeared to be recording noise in the UV-B range. These initial measurements demonstrated that we could only employ hand-held meters for field surveys if readings were corrected for measurement errors resulting from stray light and noise. [9] Correction factors for the IL-1400A meter were obtained from further, more detailed laboratory measurements of at least ten representative ILT lamp fixtures using two well-characterized, digital spectroradiometers: an International Light Technologies, ILT950 wideband (25–1050 nm) spectroradiometer SN 1007094U1 with W4 and W2 receptors, calibrated within 3 months of the measurement, with SpectrlLight III, v3.4 software; and, a Mightex HRS-UV1-025 UV (230–450 nm), calibrated within 3 months of the measurements with software v2.11. The measured spectra permitted a calculation of the effective UV irradiances for comparison with the portable meter readings and derive a correction factor for field measurements of ILTs. There was no attempt to develop correction factors for the Ocean Optics spectrometer.

Maintenance and servicing

For proper operation ILTs may require periodic cleaning and lamp replacement. Normally, the lamps are turned off during this process, but if an individual attempted to clean or replace the lamps during UV emission, the eyes could be exposed as close as ∼50 cm for as much as one minute for each lamp fixture (<30 µJ·cm⁻² at each fixture). However, even if the same individual were to service as many as 100 fixtures in a plant, the total ocular exposure would still not quite reach the daily actinic exposure limit of 3 mJ·cm⁻² = 3,000 µJ·cm⁻² and would be well below the conservative ICNIRP limit of 1.0 J·cm⁻² for direct staring at a source. [11] Hence, there is no realistic concern with regard to maintenance and service, since even by assuming unrealistic exposure conditions attempting to clean or service many units would pose no realistic hazard. Therefore, there is no justification for system safety features such as interlocks to preclude lamp operation during servicing.

Radiometric characteristics of the UV lamps

As noted earlier two BL lamp categories are common: those with a peak emission at ∼368 nm and those with a peak wavelength near 352 nm. Representative lamps from several lamp manufacturers were tested (Table 1). A highly reliable spectral irradiance measurement from Osram-Sylvania, a major supplier of these types of lamps, is shown in Figures 4A (linear presentation) and 4B (semi-logarithmic plot).

Figure 4A.

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Spectral irradiance of Osram-Sylvania F40/350BL 40-watt T12 tubular fluorescent lamp at a distance of 10 cm from the lamp tube surface. This high-quality well-calibrated spectrum was obtained using a double-monochromator with low stray-light (Model OL 750-D, Optronic Laboratories, Orlando, FL). Spectral radiance is 2 x the above vaues/sr.

Figure 4B.

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Semi-logarithmic plot of spectral irradiance of Osram-Sylvania F40T12/350BL 40-watt tubular fluorescent lamp at a distance of 10 cm from the lamp tube surface. This high-quality well-calibrated spectrum was obtained using a double-monochromator with low stray-light (Model OL 750-D, Optronic Laboratories, Orlando, FL).

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Figure 5.

Actinic S(λ)-spectrally weighted irradiances for all ILT units tested. Note that none of the units—even larger (P_electrical > 100 W) units—exceeded the daily exposure limit of 0.1 µW·cm⁻² at 200 cm.

Irradiance as a function of distance

Total UV-A irradiance as a function of distance from three representative fixtures that had maximized irradiance with new tubes for each lamp-wattage type (15, 20, and 40 W) is shown in Table 2. The actual values summarized in Table 2 are plotted as curves along with each measurement in Figures 5 and 6 on a semi-logarithmic scale to show that beyond approximately 50 cm, the irradiance decreased inversely as the square of the distance. The total UV-A irradiance was measured at a standard reference distance of 20 cm (specified in IESNA RP27.2 and RP 27.3, as well as CIE S009/IEC6247 to achieve reliable measurements of trace UV-B emissions). [9,13–15] Most lamp systems (i.e., lamps with fixtures) easily met the 1.0-mW·cm⁻² UV-A limit for the Exempt (RG0) risk group, but a few just met the 3.3-mW·cm⁻² UV-A limit for Risk Group RG-1. No ILT units exceeded the UV-A 8-h irradiance limit at distances greater than 60 cm. However, the lamps systems also had to be assessed for actinic UV—the S(λ)-weighted (largely UV-B) irradiance.​irradiance.

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Figure 6.

Non-weighted total (300–400 nm) irradiances for all ILT units tested. Note that none of the units—even larger (P_electrical > 100 W) units—exceeded the daily ACGIH and ICNIRP exposure limit of 1-mW·cm⁻² for skin at 50 cm. However, the highest-power units emitted total UV-A exceeding the more conservative ICNIRP (eyes only) limit of 33 µW·cm⁻² for 8-hr out to as much as 250 cm. Of course continuous staring at such lamps for hours is totally unrealistic.

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Figure 7.

The ACGIH UV Hazard Function S(λ) describes approximately the relative spectral risk for photokeratitis.

UV-hazard effective irradiance

The effective UV irradiance for safety assessment requires either spectroradiometric measurements that are then spectrally weighted using the UV-Hazard Spectral Weighting Function S(λ), as shown in Figure 7, or a direct-reading instrument such as the IL1400A with an SEL240 detector with a spectral response to directly record S(λ)-weighted effective irradiance. Measuring trace amounts of actinic, UV-B radiation in the presence of far greater fractions of far less biologically effective (hazardous) UV-A radiation produces uncertainties from stray light in direct-reading instruments and single-monochromator spectroradiometers, leading to over calculation of the S(λ)-weighted irradiance. Although double monochromators introduce far less uncertainty from stray light, the reduced signal-to-noise ratio frequently introduces a false signal of UV-B and UV-C from noise. In either case, the apparent measurement will be always higher than the true value. [9] We found that it was necessary to rely on the highly reliable laboratory spectroradiometric reference measurements at 10–20 cm to determine correct ratios of UV-B, UV-A, and UV-effective to the total irradiance prior to measuring effective irradiances at greater distances either with compact spectroradiometers or hazard meters. These ratios are given in Table 2.

UV-effective irradiance as a function of distance

Although the broad-band (IL SEL240) safety meter provides a reliable reading for most white-light sources, it was found to over-respond in the UV-A compared to the highly reliable spectroradiometric reference measurements used to determine ratios of UV-B, UV-A,

and UV-effective to the total irradiance measurements with the spectroradiometers. For field studies, the spectral ratios in Table 2 from the reference spectra were used to estimate the biologically effective UV-hazard irradiance as a function of distance based upon the total UV measurements presented in Table 1. The actinic UV values in Table 2 provide for an overly conservative risk assessment, since reference spectra (Figures 4A and 4B) are for new, freshly installed lamps emitting maximum irradiances. Laboratory spectroradiometric measurements were made of a wider variety of fixtures (with 100 lamps) as a function of distance as summarized in Figures 6 and 8. Table 3 lists all of the types of lamps studied in the laboratory.

Ultraviolet exposure limits

UV exposure criteria have evolved over the last several decades based upon biomedical laboratory research, human epidemiological studies, and clinical experience. [11,17–23] The current UV exposure limits are identical internationally for the skin, but differ for eye exposure durations greater than 1000 sec. The primary guideline to protect the skin and the cornea is an S(λ) weighted (Figure 7) daily (8-hr) exposure H_eff of 3 mJ·cm⁻² or 30 J·m⁻² referenced to 270 nm, which corresponds to 5 J·cm⁻² at 350 nm and 27 J·cm⁻² at 365 nm. [10,11] This S(λ) weighted daily exposure limit is also about 1/3 to 1/4 of a minimal erythemal dose for the skin and less than 1/2 the exposure necessary for clinically reported photokeratitis. The S(λ) action spectrum for safety studies was recently re-evaluated and revalidated in the 300–320 nm region, where some published threshold injury data had appeared to conflict with the exposure limits. [18] Both ACGIH and ICNIRP recommend the same limit as a guideline value for skin exposure—a goal to try to achieve, but not as a ceiling value, as it is for the eye, since the risk varies greatly with skin types. In addition to the primary S(λ)-weighted limit, a second limit that is not spectrally weighted is provided to protect the lens from excessive exposure to the UV-A: it is a 1 J·cm⁻² limit applying to all radiant energy between 315 and 400 nm. ACGIH applies this only to exposures less than 1,000 sec, and recommends a dose-rate limit of 1 mW·cm⁻² for longer exposures. However, ICNIRP applies the 1 J·cm⁻² limit to a full-day (30,000 sec) exposure. [11] It is the latter limit that is most restrictive for 360–370 nm. This is a limit that considers chronic outdoor solar exposure over a lifetime, and would be excessively conservative to apply in the context of occasional exposure to UV-A from ILT fixtures. Indeed, the ANSI and CIE standards for photobiological safety of lamps and lighting systems, consider the 1 mW·cm⁻² limit as a reasonably foreseeable worst-case time-weighted-average irradiance, which would still meet the ICNIRP limits in any one day. [11–15]

To place the exposure in perspective, it is useful to consider environmental exposure to the UV in sunlight. [19] Outdoor exposures—particularly in mid-summer—routinely exceed the ACGIH TLV of 1 mW·cm⁻², and greatly exceed the ICNIRP UV exposure guideline of 1 J·cm⁻² over a full day. [23] If one calculates the permissible exposure duration for horizontal exposure in the prone position with the eyes directed to the sun (totally unrealistic), the exposure limits may be exceeded in 5–10 min on a bright summer day at noon time. However, under realistic outdoor situations, the ocular exposure does not actually exceed the limit for even greater exposure duration of several hours, except over sand and snow. Corneal examinations of humans exposed in a desert environment for much of the day just began to show the signs of threshold photokeratitis. [20–21,23] Although an individual can readily look directly at a UV-A ILT lamp, the 1 mW·cm² limit was not achieved from any of the fixtures at distances greater than 50 cm and cumulative exposures (TWA) cumulative exposures could not even exceed the overly conservative ICNIRP limit of 1 J·cm⁻² extended to 8 hr.

Ultraviolet exposure limit formulations

The two limits that apply to near-ultraviolet exposures—a spectrally weighted limit to protect the cornea (Eq. (1)) and an un-weighted UV-A limit (Eq. (2)) intended to protect the lens and retina. [10,11] These are:

(1)

and

(2a)

and

(2b)

To calculate the maximum direct viewing duration when either (1) or (2) is not satisfied, the maximum “stare time,” t_max, is found by inverting Eqs. (1) or (2a) for a CW source with a weighted or un-weighted irradiance:

(3a)

(3b)

The ICNIRP UV-A exposure limit for the eye (not skin) would be: (1 J·cm⁻²)/(30,000 sec) = 33 µW·cm⁻²; however, as noted before this is considered greatly overly conservative to apply to any condition except staring into a source continuously, which is unrealistic for any lamp application, and therefore not applied in IESNA RP 27.3-05 or CIES009/IEC62471:2006 for defining the Exempt risk group. Sadly, an overzealous technician sometimes just ignores the concept of TWA, without thinking that the weighted irradiance limit of 0.1 µW•cm⁻² just applies to the 8-hr (30,000 sec) TWA exposure limit. Then, if applied as if relevant at the 20-cm standardized measurement distance (chosen largely to minimize stray-light measurement problems) one could reach the absurd conclusion that there is actually a hazard, although no one would be so close for more than a minute!

Blue-light photochemical retinal hazard

The ACGIH TLV [10] and ICNIRP guidelines [11] are identical for large sources and are designed to protect the human retina against photoretinitis, “the blue-light hazard” is an effective blue-light radiance L_B spectrally weighted against the Blue-Light Hazard action spectrum B(λ) and integrated for t s of 100 J·cm⁻²·sr⁻¹, for t < 10,000 sec, i.e.,

(4)

and for t > 10,000 sec (2.8 hr):

(5)

To calculate the maximum direct viewing duration when (5) is not satisfied, this maximum “stare time,” t_max, is found by inverting (4) for a CW source with a weighted radiance of L_B:

(6)

The radiance values are averaged over a field of view which is not less than 11 mrad = 0.011 rad. The blue light hazard is evaluated by mathematically weighting the spectral irradiance, E_λ, against the blue-light hazard function to obtain E_B and then converting this to a radiance L_B by dividing by the solid angle of the lamp source. As an example, using the Osram-Sylvania reference measurement, this solid angle of uniform brightness was 0.5 sr. The total, un-weighted lamp radiance was 3.6 mW·cm⁻²·sr⁻¹, and since B(λ) = 0.01 from 310–380 nm, then L_B ∼ 0.04 mW cm⁻²·sr⁻¹. Since the applicable limit Eq. (5) was 400 times greater, the retinal limits were not further checked for other lamps, since this potential hazard was clearly unrealistic.

The high-quality well-calibrated spectrum (plotted in two ways) in Figures 4a and 4b was provided courtesy of David Gross (Osram-Sylvania, Danvers, MA). Mr. Barry Grimes, an employee of Gilbert Industries, Inc., conducted some of the optical assessment testing which helped guide further studies, but did not appear in the article. Gilbert Industries, Inc., Gardener Manufacturing, Inc., and Armatron Company provided ILTs for this study.