Re-Evaluation of Rat Corneal Damage by Short-Wavelength UV Revealed Extremely Less Hazardous Property of Far-UV-C†

Abstract

Corneal damage-induced various wavelength UV (311, 254, 235, 222 and 207 nm) was evaluated in rats. For 207 and 222-UV-C, the threshold radiant exposure was between 10 000 and 15 000 mJ cm^-2 at 207 nm and between 3500 and 5000 mJ cm^-2 at 222 nm. Penetrate depth to the cornea indicated by cyclobutene pyrimidine dimer (CPD) localization immediately after irradiation was dependent on the wavelength. 311 and 254 nm UV penetrate to corneal endothelium, 235 nm UVC to the intermediate part of corneal stroma, 222 and 207 nm UVC only to the most outer layer of corneal epithelium. CPD observed in corneal epithelium irradiated by 222 nm UVC disappeared until 12 h after. The minimum dose to induce corneal damage of short-wavelength UV-C was considerably higher than the threshold limit value (TLV^® ) promulgated by American Conference of Governmental Industrial Hygienists (ACGIH). The property that explains why UV-C radiation at 207 and 222 nm is extremely less hazardous than longer UV wavelengths is the fact that this radiation only penetrates to the outermost layer of the corneal epithelium. These cells typically peel off within 24 h during the physiological turnover cycle. Hence, short-wavelength UV-C might be less hazardous to the cornea than previously considered until today.

Figure 1

(a) Spectral distributions of UV light exposed to rats. (b)(c) UV light was irradiated to the cornea at a right angle. The counter eye was closed by tape (arrow) to prevent drying.

Figure 2

Scoring of mire ring irregularity. (a) Corneal surface damage was evaluated by mire ring according to the level irregularity. No damage; score 0, slightly irregular (arrow); score 1, irregular less than 50% circle; score 2, and irregular more than 50% circle; score 3. (b) Representative cases of mire ring in each wavelength and radiant exposure.

Figure 3

Scoring of Fluorescein staining. (a) Corneal epithelial defects detected by fluorescein staining were scored according to the erosion area. No damage; score 0, superficial punctate keratitis; score 1, erosion area less than 50% of cornea; score 2 and erosion area more than 50% of cornea; score 3. (b) Representative case of fluorescein stain in each wavelength and radiant exposure. Arrows indicate superficial punctate keratitis and arrowheads indicate erosion.

Figure 4

The lowest observed adverse effect level (LOAEL) and no observed adverse effect level (NOAEL) for corneal damage according to the mire ring evaluation and fluorescein staining score. The red line and dot show LOAEL, the blue line and dot show NOAEL, and the yellow line shows the threshold limit value (TLV^®) published by American Conference of Governmental Industrial Hygienists (ACGIH) in 2020.

Figure 5

H&E staining of the cornea. (a) Unexposed normal control. (b–e) The cornea irradiated by 207‐nm UV‐C. (f–i) The cornea irradiated by 222‐nm UV‐C. (j–l) The cornea irradiated by 235‐nm UV‐C. (m–o) The cornea irradiated by 254‐nm UV‐C. (p–r) The cornea irradiated by 311‐nm UV‐B. The small box indicates the high‐magnification images of the part surrounded by the blue line. Arrow indicates exposed stroma. Bar = 50 µm in each panel and 25 µm in the high‐magnification images.

Figure 6

Expression of CPDs in the cornea. (a) Unexposed normal control. (b) Negative control. (c–f) The cornea irradiated by 207‐nm UV‐C. (g–j) The cornea irradiated by 222‐nm UV‐C. (k–m) The cornea irradiated by 235‐nm UV‐C. (n–p) The cornea irradiated by 254‐nm UV‐C. (q–s) The cornea irradiated by 311‐nm UV‐B. Small box indicates the high‐magnification images of the part surrounded by the blue line. Bar = 50 µm in each panel and 25 µm in high‐magnification images.

Figure 7

(a) CPD localization in the cornea immediately after UV irradiation. Arrows indicate PCD‐positive cells. Bar = 100 µm in the upper panel and 25 µm in the lower panel. (b) CPD localization in corneas of the various animals immediately after 222 nm or 254 nm in the UV‐C band. The part surrounded by a black line is indicated below with high magnification. Bar = 100 µm in the upper panel and 25 µm in the high‐magnification images.

Figure 8

The change of CPD localization in the cornea when irradiated by 254‐nm (a–f) or 222‐nm UV‐C (h–m). (g) Negative control. Arrows indicate PCD‐positive cells. Bar = 50 µm in each panel.