Determining the Lethal Dose of UV-C Light Required for Inactivating SARS Cov-2 in Commercial HVAC Systems

Warren Jackson, Founder – Wij Tech Projects, LLC

Summary

Removal of pathogens via UV-C light inactivation and/or filtration is only one of three (3) important steps in providing safe, virus free air in indoor spaces.  For an extremely contagious aerosol like Sars Cov-2, decreasing HVAC turbulence to allow high virus capture efficiency is just as important if not more, than the strength of the UV-C light system.

AIRUSOL™ is the only system available on the market today that excels in all three (3) steps for making clean, fresh, and virus free air for commercial spaces.  Take a free virtual tour of AIRUSOL™ TODAY at WijTechProjects.com > Services > Indoor Air Quality.

Introduction

UV-C light at 254 nm has been used for over a century in medical settings because of its known effectiveness for inactivating viruses and bacteria. Interesting UV-C light for HVAC systems was previously limited to benefits gleaned from keeping cooling coils clean in air handler units.  However, the 2020 pandemic put a spotlight on our need to expand the use of UV-C light in HVAC systems for room disinfecting purposes.  Initially there was only a scintilla of data available on the new coronavirus,but today we have enough physical properties of SARS Cov-2 and other pathogens to design commercial HVAC systems that can limit the indoor transmission of disease.  In this paper you will see how laboratory data from independent 3^rd party testing was used to determine by scale-up, the lethal UV-C light dose delivered by WIJ Tech’s patented AIRUSOL™ disinfecting HVAC system.

1. S. Heilingloh et Al. – The German Study

The data used for scale-up comes from the work performed by C. S. Heilingloh et al. early during the pandemic and published in the American Journal of Infection Control48 (2020) 1273-1275.A SARS Cov-2 sample was taken from the nasal passage of a covid patient who was hospitalized at the Department of Infectious Diseases at the University Hospital Essen.   The sample was properly incubated, cells harvested, and the 50% tissue culture infective dose (TCID₅₀) was calculated.  In this study UV-C light at 254 nm was maintained at 3 cm above the bottom of the well plates containing SARS Cov-2.  The UV-C light intensity was held at1940 µW/cm².  Samples were taken over time, frozen, and later analyzed for live virus.

The experimental tests results for this UV-C/SARS Cov-2 study are reported in a X-Y semi-log plot as Viral Load versus UV-C Light Dose.The system inactivation constant does not change, and the UV-C light intensity is also constant during the experiment.

Therefore, time is the independent variable, and the biological decay function governing inactivation of micro-organisms is:

S(t)/S_o= e ^{– ( k I t)}

• Where S_o is the initial concentration of live virus at time t=0,
• S(t) is the amount of live virus at any time t,
• k is the inactivation constant for the UV-C / SARS Cov-2 system, and
• I is the UV-C light intensity.

Data Analysis

Taking the natural log of both sides of the decay function above yields:

ln(S) = -k I t + ln(S_o)

Hence, if we plot the natural log of the live virus concentration versus time, we get a straight line whose slope is the product of k, the UV-C / SARS Cov-2 system inactivation constant,and the UV-C light intensity.  Since the light intensity is known, we can extract the inactivation constant and use it for scale-up to commercial HVAC systems.

Hence, the inactivation constant for the system is in the slope of the decay line:

k I = -(1.4286 TCID₅₀/ml )/min

k = [( -1.4286 TCID₅₀/ml)/min ] / [ 1940 µW/cm² ] = -736.39 TCID₅₀/cm/min/µW

Now with this k value, we can use the regression equation to scale-up from laboratory strength UV-C light intensity to something more suitable for commercial application; 25W mercury vacuum tube lamps are readily available, come with reflectors, and reliably produce UV-C light at 254 nm.  Let’s ignore the distance and light angle effects as a first approximation, and project the 25 Watt UV-C light onto an imaginary 25 x 25 square inch plane that is perpendicular to air flow inside an air handler.  The commercial light intensity for 25 Watts is then calculated to be:

I = 6200 µW/cm²

For 99.9% virus reduction in a commercial HVAC unit, we can calculate the time required for inactivation:

ln (S) =- k I (t) + ln (S_o)

ln(.0122TCID₅₀/ml) = – (736.39TCID₅₀/cm/min/µW )( 6200µW/cm² )( t ) + ln(12.2 TCID₅₀/ml)

And the virus inactivation time is:

t = 1.5 x 10^-6minutes (60)s/min =9×10^-5 seconds

Hence, a 25W UV-C lamp kills SARS Cov-2 instantaneously.

Finally, let’s consider how much time the air stream being treated inside the air handler is exposed to the UV-C light, and see if this time is long enough for the virus to be inactivated.  If we take three (3) inch thick slice of our 25 x 25 square inch air handler, then the reactor volume for inactivation is:

V_rxtr = 1.085 ft³

Using the CFSTR model, the holdup time  is the reactor volume divided by the

volumetric air flow rate Q through the air handler:

= V_rxtr /  Q   =  (1.085ft³) (60 s/min) / [ 2000 ft³/min ] = 0.03 seconds

Discussion

The holdup time for 2000 ft³/min of air passing through a three (3) inch slice (Δ L) of a typical air handler is0.03 seconds, compared to an instantaneous virus inactivation that takes place in 9×10^-5 seconds by using a 25 Watt UV-C mercury vacuum tube lamp.  The virus inactivation happens so quickly that going back and adjusting the 25W light intensity for distance and angle would not alter our 1^st impressions and conclusions.

In using the CFSTR model we are assuming that the reactor is well mixed, and that the amount oftime a virus particle is shielded from the UV-C light source by another virus particle is neglectable. The reaction zone inside of the air handler is indeed well mixed based on a Reynolds number on the order of 10⁵, which reduces the probability of shielding occurring.  Additional conservatism comes from the fact that in the laboratory experiment there very well could have been some cell shielding in the 24-well plate.  This would give a longer time for 99.9% inactivation in the laboratory experiment, and this additional time although not needed, would be included in the commercial scale-up.

Conclusions

UV-C light is very effective for inactivating viruses like SARS Cov-2 in commercial HVAC systems.  Installing a 25W germicide light in an open area of a typical air handler or even inside large duct work is sufficient.   However, light installation with focus toward the coil and drain pan has two (2) advantages.  In addition to keeping the coils clean, the lamp is in a much smaller cross-sectionalarea of the air handler whichcompresses the total air stream into closer proximity to the UV-C light source for greater intensity and a faster kill.