Disinfection of Phi6, MS2, and Escherichia coli by Natural Sunlight on Healthcare Critical Surfaces

Ultraviolet (UV) radiation systems, commonly used to disinfect surfaces, drinking water, and air, stem from historical practice to use sunlight to disinfect household items after contagious illness. Currently, it is still recommended in viral outbreak contexts such as COVID-19, Ebola, and Marburg to expose soft surfaces to sunlight after washing with detergent or disinfecting with chlorine. However, sunlight that reaches the Earth's surface is in the UVA/UVB wavelengths, whereas UV disinfection systems typically rely on biocidal UVC. Our goal was to fill the evidence gap on the efficacy of sunlight disinfection on surface materials common in low-resource healthcare settings by seeding four surfaces (stainless steel, nitrile, tarp, cloth) with three microorganisms (viral surrogate bacteriophages Phi6 and MS2 and Escherichia coli bacteria), with and without soil load, and exposing to three sunlight conditions (full sun, partial sun, cloudy). We conducted 144 tests in triplicate and found: solar radiation averaged 737 W/m2 (SD = 333), 519 W/m2 (SD = 65), and 149 W/m2 (SD = 24) for full sun, partial sun, and cloudy conditions; significantly more surfaces averaged ≥ 4 log10 reduction value (LRV) for Phi6 than MS2 and E. coli (P < 0.001) after full sun exposure, and no samples achieved ≥ 4 LRV for partial sun or cloudy conditions. On the basis of our results, we recommend no change to current protocols of disinfecting materials first with a 0.5% chlorine solution then moving to sunlight to dry. Additional field-based research is recommended to understand sunlight disinfection efficacy against pathogenic organisms on healthcare relevant surfaces during actual outbreak contexts.