Intermittent low-dose far-UVC irradiation inhibits growth of common mold below threshold limit value

Abstract

Mold infestations in buildings pose significant challenges to human health, affecting both private residences and hospitals. While molds commonly trigger asthma and allergies in the immunocompetent, they can cause life-threatening diseases in the immunocompromised. Currently, there is an unmet need for new strategies to reduce or prevent mold infestations. Far-UVC technology can inactivate microorganisms while remaining safe for humans. This study investigates the inhibitory efficacy of far-UVC light at 222 nm on the growth of common mold-producing fungi, specifically Penicillium candidum, when delivered in low-dose on-off duty cycles, a configuration consistent with its use in real-world settings. The inhibitory effect of the low-dose duty cycles was assessed on growth induced by i) an adjacent spore-producing P. candidum donor and ii) P. candidum spores seeded directly onto agar plates. In both setups, the far-UVC light significantly inhibited both vertical and horizontal growth of P. candidum, even when the UV doses were below the Threshold Value Limit of 23 mJ/cm2. These results suggest that far-UVC light holds the potential to improve indoor air quality by reducing or preventing mold growth, also when people are present.

Fig 1. Far-UVC inhibits P. candidum growth induced by airborne spores.

A donor plate covered with P. candidum was placed in a box with 80% humidity, air movement, and a far-UVC lamp, and recipient plates were positioned adjacently to the donor plate (A). Accumulated doses of cycling far-UVC light were 25, 50, and 100 mJ/cm² per 24 hours. Representative pictures show fungal growth on the recipient agar plates with and without far-UVC light (B). The growth was monitored by measuring the percentage of the agar surface covered by mycelium (C). The mean percentage values for surface coverage were 21.7% (control), <1% (25 mJ/cm²), 0% (50 mJ/cm²), and 0% (100 mJ/cm²). Statistical analysis was performed using unpaired Welch’s t-test, with the asterisks indicating a p-value of 0.0005. The p-values for 50, and 100 mJ/cm² conditions versus control were both 0.0005 (not shown in the figure). Columns indicate mean values with standard deviation, and each dot represents a recipient agar plate. The results indicate that cycling far-UVC light can inhibit mold growth, even when the dose is only 25 mJ/cm² per 24 hours.

Fig 2. Far-UVC inhibits both vertical and horizontal growth of P. candidum.

P. candidum spores were seeded onto agar plates and left to grow in 80–90% humidity for seven days while being exposed to far-UVC light (A). Growth was monitored daily by measuring diameter increases (B, C) and through visual assessment (C). After seven days, the mean diameter increases were 3.28 ± 0.96, 0.53 ± 0.42, 0.25 ± 0.30 and 0.0 mm for control, 10, 15, and 20 mJ/cm², respectively. Growth in the non-irradiated control condition was significantly higher (Welch’s unpaired t-test p-value <0.001) compared to any of the irradiated samples. Representative images (C) show notable hyphal growth (indicated by white arrows) from day two under non-irradiated control conditions, which is absent in all the of the far-UVC irradiated samples. This indicates that cycling doses of far-UVC below the TLV inhibit the growth of P. candidum.