Verification of inactivation effect of deep-ultraviolet LEDs on bacteria and viruses, and consideration of effective irradiation methods

DNA (deoxyribonucleic acid) of bacteria and RNA (ribonucleic acid) of viruses have a maximum absorption wavelength around 260 nm. ²⁾ When the light of the same wavelength is irradiated on them, dimers are formed (thymine is a typical example), which leads to the loss of the nucleic acid replication function, killing the bacterium or inactivating the virus. ³⁾ For this reason, mercury lamps with a peak wavelength of 254 nm have been used to date as a light source for sterilization and inactivation of viruses. ^4,5) The effect of inactivation on viruses is proportional to the photon energy, and the fungal survival rate S can be obtained by Eq. (1).

$$\begin{eqnarray}&&S=\displaystyle \frac{P}{{P}_{0}}={e}^{\displaystyle \frac{-Et}{Q}}\end{eqnarray} \tag{ 1 }$$

(Number of bacteria before UV irradiation P₀ , number of bacteria after UV irradiation P, effective UV illuminance E (mW/cm²), irradiation time t (sec), UV dose required to set the survival rate S to 1/e = 36.8% Q)

The ultraviolet dose required to achieve a 99.9% sterilization or virus inactivation is available in data accumulated from papers, and applications are developed based on these data. ^6–9) Figure 1 shows the spectra of a UV lamp and a 275 nm UVC-LED. The spectrum of a UV lamp spread widely from 180 to 580 nm and exhibits several small peaks other than the sterilization wavelength. On the other hand, 275 nm UVC-LED has a single spectrum and narrow range of full width at half maximum around 11 nm.

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Several groups have already reported that mercury lamps and UVC-LEDs with wavelengths of 265–285 nm are effective in inactivating SARS-CoV-2. ^11–14) We have verified the inactivation of human coronaviruses (HCoV-229E), which have S-protein spikes on their surfaces and belong to Coronaviridae the same family as SARS-CoV-2.

Since it is difficult to obtain the irradiation dose required to sterilize or inactivate with UVC-LED around 254 nm, we verified whether 275 nm UVC-LED ¹⁵⁾ is effective to inactivation on HCoV-229E, which has practical emission intensity.

2.1. Verification method

2.2. Results

By irradiating influenza viruses with three different wavelength types of UVC-LEDs, we examined the inactivation effect of different wavelengths on influenza viruses attached to solid surfaces.

3.1. Verification method

3.2. Results

4.1. Verification method

We verified the inactivation effect by irradiating Legionella pneumophila in water with four different types of waterproofed UVC-LEDs by using a rectangular tank. Four kinds of peak wavelengths and optical outputs are 265 nm (1.5 mW), 275 nm (1.5 mW), 285 nm (1.5 mW), and 275 nm (3.5 mW), the directivity angle around 120° and full width at half maximum around 10 nm. A UVC-LED unit was attached to one end of the water tank (Fig. 2) and the tank was filled with water. The UVC-LED unit is in contact with water.

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Then, water and Legionella pneumophila filled in a quartz container were placed in the tank. After irradiating for 30 min, the number of Legionella pneumophila were counted to evaluate the sterilization effects.

4.2. Results

It has become clear that the main route of infection of COVID-19 is an aerial infection by aerosols. The size of aerosols, which is a droplet containing viruses floating in the air, varies from 0.001 to 100 μm. ¹⁶⁾ A cough generates aerosols of 10⁵ to10⁷. ¹⁷⁾ Therefore, we devised an air purifier comprising UVC-LEDs and investigated the effects of the two mechanisms of virus inactivation using UV irradiation and virus removal by filter units on controlling aerosols containing viruses.

5.1. Verification method

LEDPURE AM1 (Fig. 3) air purifier, which has the following structure, a polyester Pre-filter that captures large droplets of 30 μm or more, a non-woven HEPA filter that captures ultrafine particles of 0.3 μm or more, and a honeycomb ceramics filter that supports titanium dioxide (photocatalyst). Four UVC-LED lamps of 275 nm and 2 mW irradiate the surface of the HEPA filter from 10 mm distance, 12 UVA-LED lamps of 365 nm and 350 mW irradiate photocatalyst from 10 mm distance. The air purifier was operated in an environmental test booth of 1.2 m³ maintained at air cleanliness class 10 to examine the reducing effect of active airborne influenza viruses in an experimental system dedicated to virus evaluation.

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5.2. Results

When the HEPA filter was removed and only UVC-LED was irradiated, a very low reduction of active viruses in the air was observed. It gave similar results when the HEPA filter was removed and UVA and UVC-LED were not irradiated. Next, when the HEPA filter was removed and UVA-LED and UVC-LED were irradiated, the active virus was no longer detected in 50 min. This suggests that the UVA-LED works on the photocatalyst and exhibits an inactivating effect. Finally, when the HEPA filter was attached and UVA-LED and UVC-LED were irradiated, the active virus was no longer detected before 30 min passed (Fig. 4). We confirm that the combination of the UVC-LED and the HEPA filter most effectively reduces the amount of active viruses in the aerosols in the atmosphere.

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The external quantum efficiency of UVC-LEDs is as low as 2.6% in our mass-produced products with a wavelength of 275 nm. Also, since the products adopt a point light source, it is not suitable for inactivating a large area. ¹⁸⁾ Therefore, to obtain higher light energy, we fabricated a 290 mW (IF: 800 mA) module (NS275B-44FA) with 16 UVC-LED chips mounted on a high heat dissipation substrate. By applying NS275B-44FA, we fabricated a floodlight (IF: 600 mA) with an aluminum reflector as shown in Fig. 5(a) and examined illuminance and illuminance distributions at irradiation distances (Vd) of 5 cm and 20 cm, respectively [Fig. 5(b)]. In the irradiated area of 30 × 30 cm, the case of Vd 5 cm shows an uneven illuminance distribution, while the case of Vd 20 cm shows a uniform distribution with an illuminance higher than almost 80% (Fig. 6).

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Following this, we evaluated the inactivation of HCoV-229E using this floodlight. At a distance of 10 cm, more than 99.9% of active viruses in 10 × 10 cm area were inactivated within 1 min and 30 s. At a distance of 30 cm, more than 99.9% of active viruses in a 30 × 30 cm area were inactivated in 15 min, and 99.0% of the active viruses in 60 × 60 cm area were also inactivated. At a distance of 50 cm, more than 99.9% of active viruses in 50 × 50 cm area were inactivated in 40 min, and 99.68% of viruses in 100 × 100 cm area were inactivated.

From these results, we can find a correlation between illuminance and inactivation. For efficient inactivation, optimization of the specification and location of the light source in each space is important. In addition, since deep-ultraviolet wavelengths have an effect on the human body, ^19,20) it is necessary to design a space that can be sterilized taking these factors into account.

Since irradiation of ultraviolet light of wavelength 253.7 nm causes inflammation of eyes and skin, as well as cataracts and skin cancer, according to the Japanese Industrial Standard JISZ8812, threshold limit values should be kept within 8 h and below 60 J m⁻² for a period of 1 day (24 h). Furthermore, the allowable limit value is set at 30 J m⁻² for a wavelength of 270 nm. Table II shows the illuminance of our UVC-LED products and the allowable irradiation time according to JISZ8812. NS275L-6DIG, a surface-mounted LED device with one 275 nm chip, has the smallest output power of 3.1 mW, but the allowable irradiation time is 0.6 s at 0 cm above the surface. In the case of the floodlight with NS275B-44FA (Fig. 5), the irradiation time must be kept below 3 min and 4 s even at a distance of 50 cm, indicating that the UVC-LED has a high illuminance and a large impact on the human body.

Our group has developed a UVC-LED shower box to sterilize and inactivate bacteria and viruses attached to the entire surface of protective clothing, while a health care professional for SARS-CoV-2 wears the protective clothing. The shower box is equipped with 24 floodlights with NS275B-44FA. Direct irradiation to the human body is dangerous, as shown in Table II. However, the illuminance on the body surface can be suppressed below the detection limit (0.5%) using protective clothing, mask, and face shield. Figure 7 shows a simulation of the irradiation area. The operator can inactivate the viruses attached to the whole body in 4 min and 20 s by receiving irradiation from the front, back, and both sides of the body in the shower box.

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UVC-LEDs are effective for sterilization or virus inactivation on surfaces, in water, in aerosols, in all environments, depending on how they are used. Unlike mercury lamps, UVC-LEDs do not require warm-up time, and can be driven by pulses. ²¹⁾ The products can be controlled to switch ON and OFF according to the status of people entering and leaving the room, which is detected by motion sensors. Even when entering the room, by controlling the current value, adverse effects on the human body can be minimized while controlling the risk of virus infection in the area. Due to the widespread of SARS-CoV-2, it is necessary to build infection prevention systems that are safe for human health without stopping the flow of people. We sincerely hope that the environmentally friendly UVC-LEDs will be applied correctly to curb the threat of viruses.