A Brief Summary of the Investigation Report on the UVC-LED Deep Ultraviolet Dose Required to Inactivate the 2019-nCoV Novel Coronavirus
Abstract: Based on the UV inactivation doses reported in the literature for coronaviruses of the genus Betacoronavirus, such as SARS and MERS, the 99.991 TP3T UV inactivation dose for the 2019 -nCoV NCP, with an initial TCID₅₀ concentration of approximately 10⁵.⁸ per millilitre, is 1,085 mJ/cm². A basic model for deep-ultraviolet LEDs, which can serve as a reference for surface disinfection and air purification, is provided to aid in the development of various application products.
1. Background
It has now been more than two months since the outbreak of the novel coronavirus in Wuhan, and substantial medical resources have been devoted to patient admission, the development of treatment methods, the screening of drug efficacy, and the prevention and control of the spread of the outbreak. The “Treatment Protocol for Pneumonia Caused by Novel Coronavirus Infection (Trial Version 4)” [1], published by the National Health Commission, states: ‘Our understanding of the physicochemical properties of coronaviruses is largely derived from research into SARS-CoV and MERS-CoV. The virus is sensitive to ultraviolet light and heat; exposure to 56°C for 30 minutes, or to ether… can effectively inactivate the virus…”. These physicochemical properties of coronaviruses make it feasible to use ultraviolet (UV) disinfection technology to eliminate coronaviruses on various surfaces, in the air and in water; as a result, a large number of UV devices have been deployed in environments with high viral concentrations, such as hospitals [2, 3]. Industry news on UV LEDs has also reported that a research team led by Dong Xiaoping, an expert at the National Institute for Viral Disease Control and Prevention under the Chinese Centre for Disease Control and Prevention, found that UVC irradiation at an intensity greater than 90 μW/cm² can inactivate the SARS virus within 30 minutes [4]. To date, no more quantitative statements or experimental data regarding the UV lethal dose for the novel coronavirus (2019-nCoV) have been reported. However, in the practical application of UV technology, the application environment, target inactivation rate, UV intensity and dose are technical parameters that must be taken into account. The lack of quantitative experimental data means that the application of UV lamps or UV LEDs will remain at a qualitative or empirical level.
Of course, due to limited experimental resources, there are significant difficulties in actually conducting studies on the UV inactivation dose and kinetics of the novel coronavirus 2019-nCoV. Consequently, during the SARS epidemic, research on the Mouse Hepatitis Virus (MHV) provided an excellent scientific foundation for the discovery and identification of SARS, as well as for understanding its physicochemical properties. It is understood [5] that coronaviruses comprise four genera: Alphacoronavirus, Betacoronavirus, Deltacoronavirus and Gammacoronavirus. The highly pathogenic coronaviruses SARS-CoV and MERS-CoV, as well as MHV—a model virus used to study the molecular virology of coronaviruses—all belong to the Betacoronavirus genus. The genome of this virus consists of a single-stranded positive-sense RNA approximately 30,000 nucleotides in length, making it one of the RNA viruses with the largest genomes. MHV, SARS-CoV, MERS-CoV and NCP 2019-nCoV exhibit a high degree of homology and structural similarity; consequently, their resistance to ultraviolet light is comparable and serves as a point of reference for one another.
Based on the above facts, this paper reviews research data from selected literature on the inactivation of various coronaviruses, such as SARS-CoV and MHV-CoV, by ultraviolet radiation. It summarises the UVC doses required for inactivating viruses on surfaces and in the air, as well as the influencing factors, with a view to guiding the development of UVC-LED applications. The specific virus culture and experimental procedures are not the focus of this paper; these can be found in the references and will not be elaborated upon here. Furthermore, given that deep-ultraviolet UVC-LEDs were not yet a widespread technology in the year the references were published, the light sources used were all ultraviolet mercury lamps; however, this does not mean that the findings are of no relevance to the design of deep-ultraviolet UVC-LED solutions. Furthermore, as the inactivation efficiency of 275 nm deep-ultraviolet LEDs is identical to that of 254 nm ultraviolet lamps, the inactivation data for the lamps can be extrapolated to deep-ultraviolet LEDs.
2. The UV dose required to inactivate coronaviruses on surfaces
Studies conducted by Taylor’s team at the US Food and Drug Administration’s Centre for Bioevaluation and Research in 2004 [6] and 2006 [7] showed that, when exposed to a deep ultraviolet light source with a characteristic wavelength of 254 nm and an irradiance of 4016 μW/cm², SARS-CoV samples with an initial TCID50 concentration of approximately 10⁵.⁸/mL were partially inactivated after 1 minute; after 6 minutes, the number of surviving particles was ~10/mL, with an inactivation rate of over 99.99%; after 10 minutes, the concentration was ≤1.0 TCID₅₀ (log₁₀) /ml, and the inactivation rate can be considered to be >99.9999%. The table also shows that the log value of the inactivation rate is positively correlated with the cumulative irradiation dose (in this case, the irradiation time), and that the relationship is linear until the log value falls below 4. Therefore, at a viral concentration of 10⁵.⁸ /mL, the UV dose required to achieve a 99.991 TP3T inactivation rate is 1,445 mJ/cm². Based on the linear relationship between the log value of the inactivation rate and the dose, it is not difficult to determine that the dose required to achieve a 99.91 TP3T inactivation rate is 722 mJ/cm². Whilst UVA can combine with photocatalysts to produce reactive oxygen species and thereby disrupt the viral RNA chain, UVA ultraviolet radiation (365 nm, 2133 μW/cm²) has no inactivating effect on the SARS-CoV coronavirus.
In 2003, Dong Xiaoping’s team [8] reported that UVC irradiation of coronaviruses at an intensity of >90 μW/cm² could inactivate the SARS virus within 60 minutes (note that the original text states 60 min, rather than the 30 min mentioned in earlier news reports), which corresponds to a cumulative dose of 324 mJ/cm², lower than the 1,445 mJ/cm² reported by Taylor’s team. This may be due to differences in experimental conditions or the source of the viral samples; furthermore, as Dong Xiaoping’s team did not provide specific log values, there is a five-fold discrepancy (the log value reflected in the inactivation rate is only 0.5) . From the perspective of performance redundancy, it is recommended that manufacturers, when designing their solutions, base their calculations on a dose of 1085 mJ/cm² corresponding to a 99.991 TP3T inactivation rate.
Following the spread of the epidemic earlier this month, based on the lethal dose of the novel coronavirus as summarised in several domestic and international literature reviews, the initial concentration was approximately 105.8/mL. The predicted UV lethal dose required to achieve a 99.991 TP3T inactivation rate was calculated to be 1,445 mJ/cm². This dose is inaccurate and requires correction. It was calculated by multiplying the optical power density by time. This line of reasoning overlooks a crucial detail: the SARS test scenario involved an aqueous solution 1 cm deep, and it is inaccurate to calculate the irradiation dose for the entire solution based on a surface intensity of 4016 μW/cm². Irradiance is inversely proportional to the square of the distance. Consequently, the irradiance at the bottom and top of the solution is not the same. When calculating the cumulative dose, a linear average should be taken. As LEDs emit light from a small area, approximating a point source, the distribution across the irradiated surface is non-uniform; however, given that fluctuations within a 1 cm range are minimal and have little impact, an average can be approximated here. Furthermore, regarding the absorption of UVC by aqueous solutions, previous studies have indicated that pure water exhibits relatively low absorption at 275 nm, and given the limited density of virus dispersion, the absorption by water can be disregarded here. Following integration over depth, we hereby formally revise the lethal dose for the novel coronavirus to 1085 mJ/cm². We invite our colleagues in the industry to refer to this figure.
The SARS-CoV virus used in the inactivation experiments described above was dispersed in a phosphate solution, which simulates conditions on surfaces and in water. Of course, it has been demonstrated that coronaviruses in tissue fluids, such as blood, cannot be inactivated directly by ultraviolet radiation [7].
It is worth noting, however, that gamma rays—which are capable of damaging both normal human cells and cancer cells (at doses as high as 15,000 rad)—have virtually no effect on the SARS virus.
3. The UV dose required to inactivate coronaviruses in the air
In a single-cycle air duct measuring 50 × 260 × 455 mm (height × width × length), with an air flow rate of 12.5 L/min, air containing an aerosolised viral concentration of 10⁴–10⁵ PFU/mL and a humidity of 50%Rh was introduced. A 36 W UV mercury lamp (ozone-free) was used to irradiate the duct from the 455 mm end, with the UV radiation intensity at the window adjusted to 599 μW/cm². After 15 minutes of stable operation, sampling at the single-cycle air outlet revealed an inactivation rate of 87.8% for the MHV coronavirus compared with the control group. The data in Table 2 also indicate that the inactivation rate of viral aerosols is higher under high humidity conditions.
The above data provides empirical support for air disinfection scenarios. It can be roughly calculated that, in recirculating air mode, to achieve an MHV coronavirus inactivation rate of 87.81 TP3T in a 30-cubic-metre space over a two-hour period, 320 pieces of 10 mW, 275 nm UVC-LED chips are required, which is far greater than the number of LEDs required in natural microbial scenarios (theoretical calculations indicate 2–4 pieces [10], depending on product design). The surface inactivation dose for E. coli is approximately 13.2 mJ/cm² (99.991 TP3T); the dose required for the SARS virus, as outlined in Section 2 of this paper, is 110 times that value, which is broadly consistent with the dose multiplier required for air purification. Therefore, for practical application, it is necessary to set the UVC-LEDs to operate continuously for 24 hours; in this case, a space of 30 cubic metres would require 27 UVC-LEDs, which is already within the cost requirements for commercial applications. It is reasonable to speculate that this dose relationship may also apply to both still and flowing water.
4. The significance of reference values for the UVC ultraviolet inactivation dose of the novel coronavirus (NCP 2019-nCoV)
Experimental data indicate that heating SARS-CoV at 56°C for 20 minutes achieves an inactivation rate of 99.9991 TP3T or higher, and that the time required can be reduced to as little as 10 minutes at 65°C. Although there are reports suggesting that NCP 2019-nCoV can be inactivated by heating at 56°C for 30 minutes, thermal resistance and UV resistance are not comparable. Therefore, the physicochemical properties of SARS cannot be used to scientifically infer the UV resistance of NCP 2019-nCoV and should only be treated as a reference. Kurt Bedell of the University of Iowa, BS reported that, at a distance of 1.22 metres, the inactivation times for MHV-A59 and MERS-CoV—both belonging to the Betacoronavirus genus—at a concentration of 8.9 × 10⁵ pfu/mL were 10 minutes and 5 minutes respectively, representing a two-fold difference in the lethal dose [11]. However, as mentioned earlier, the difference between two-fold doses corresponds to a LOG value of only 0.3 in the inactivation rate; given the high LOG base, this equates to a difference between 99.91 TP3T and 99.951 TP3T, which is negligible in production.
5. Conclusion
With the virus raging, companies in the deep ultraviolet LED industry have a moral obligation to go further in developing effective solutions and supplying core components. We hope this article will serve as a starting point; we trust that the 1085 mJ/cm² 99.991 TP3T disinfection lethal dose (with an initial viral TCID50 concentration of approximately 10⁵.⁸ /mL) and the corresponding surface disinfection and air purification models described herein will assist in the development of various deep-ultraviolet LED applications. We look forward to joining forces with like-minded manufacturers to combat the novel coronavirus pandemic together.
Further information
6. References
[1] National Health Commission. Treatment Guidelines for Pneumonia Caused by Novel Coronavirus (Provisional, Fourth Edition) [Z]. 2020.
[2] LEDinside. Key facts to know about UV sterilisation and disinfection during the pandemic! [EB/OL]. https://www.ledinside.cn/news/20200204-46872.html.
[3] China.com News. What lessons can we learn from SARS in the fight against the novel coronavirus? [EB/OL]. https://news.china.com/socialgd/10000169/20200121/37728369_all.html#page_2.
[4] Darnell M E R, Subbarao K, Feinstone S M, et al. Inactivation of the coronavirus that causes severe acute respiratory syndrome, SARS-CoV[J]. Journal of Virological Methods, 2004, 121:85–91.
[5] Darnell M E, Taylor D R. Evaluation of inactivation methods for severe acute respiratory syndrome coronavirus in non-cellular blood products[J]. Transfusion, 2006, 46(10):1770–1777.
[6] Duan S M, Zhao X S, Wen R F, et al. Stability of the SARS coronavirus in human specimens and the environment, and its sensitivity to heating and UV irradiation[J]. Biomed Environ Sci, 2003, 16(3): 246–255.
[7] Walker C M, Ko G. Effect of Ultraviolet Germicidal Irradiation on Viral Aerosols[J]. Environment Science & Technology, 2007, 41(15).
[8] Kurt Bedell B, Buchaklian A H, Stanley Perlman M. Efficacy of an Automated Multiple Emitter Whole-Room Ultraviolet-C Disinfection System Against Coronaviruses MHV and MERS-CoV[J]. Infection Control & Hospital Epidemiology, 2016, 37(5):598–599.
[9]References from DUVTEK Edited by Shenzhen Deshengxing Electronics Co., Ltd.www.uvstry.cn
How do you measure the UV index and UV intensity? The obvious choice is, of course,EIT Power Puck II (USA)
