UV Disinfection Fundamentals

UV Disinfection Fundamentals

Chris RockettDirector of Engineering

Light Sources Inc.

El Paso

Gainesville

Atlanta

Milford

• Born 1981 in Gainesville, Florida

• Family moved to El Paso, Texas in 1983 where I grew up and graduated public high school in 2000

• Moved to Atlanta in 2000 to attend Georgia Tech• BS & MS degrees in materials science &

engineering, 2000-2007

• Currently living in Milford, Connecticut

• Outside of work, hobbies:• Guitar/music: played guitar since 1990, semi-

professional since 2003, can also sing and play some drums & piano

• Travel: I’ve visited ~26 countries and ~36 US states

• Upper-intermediate speaker of Chinese: three years of classes at GT, six years of private study

• Fitness: bicycling, running, yoga, weightlifting

• Married with no children, a cat & dog, wife is a psychologist

About Me

Ultraviolet (UV) radiation is part of the electromagnetic spectrum, just shorter in wavelength than visible light

Sources of Ultraviolet Radiation

Thermal radiation sources: the Sun, welding arcs

Gas-discharge sources: mercury vapor, xenon, & excimer lamps

Solid-state sources: UV-LED’s (light-emitting diodes)

Gas-discharge lamps are currently the most powerful, efficient, economical, and long-lived source of UV radiation, so we will limit our discussion primarily to low-pressure mercury lamps.

• These are tubes made of special high-purity quartz glass (SiO2) with metal filament assemblies sealed into each end, filled with a low pressure of inert gas (usually argon) and a small dose of mercury (<15 mg)

• Electric current is passed through the gas and causes electronic excitation of mercury atoms. The excited electrons fall from a higher energy level to their ground state and in doing so emit a photon whose energy is equal to the difference in energy between the two states.

• The strongest emission of mercury is at a wavelength of 254 nm, which falls in the UVC band of the ultraviolet spectrum. It happens to match up closely with the peak UV-sensitivity of DNA

Fluorescent lighting lamps are also low-pressure mercury ultraviolet lamps, but an internal coating of phosphor material converts the UV emission into visible light.

Photo-induced DNA damage is the basis for UV disinfection

Note: this DNA damage does not kill the organism. Rather, it prevents the replication and reproduction processes such that the organism loses the ability to infect. Therefore, we prefer the term inactivation of microorganisms. Nevertheless, the terms “kill” and “germicidal” are used liberally in this industry.

• Radiant Flux

• Optical power, measured in watts [W]

• 1 W = 1 J/s

• Can be thought of as the number of photons emitted by a source per unit time, multiplied by the energy

of each photon

• Irradiance

• Radiant flux incident upon a defined surface area, measured in watts per square meter [W/m2]. The

scientifically rigorous definition of “intensity”.

• Can be thought of as the number of photons hitting a defined surface area per unit time, multiplied by

the energy of each photon

• In practice, mW/cm2 is a more convenient unit

• Must always be stated at a specified measurement distance!

• Dose (or Fluence)

• Accumulated irradiation energy absorbed by a surface or particle, the product of irradiance multiplied

by exposure time, measured in joules per square meter [J/m2]

• Can be thought of as the number of photons that hit a surface in a given exposure time, multiplied by

the energy of each photon

• Again, mJ/cm2 is more practical

Radiometric Terminology

Dose [mJ/cm²] = Irradiance [mW/cm²] x Exposure Time [s]

(Remember: dependent on the power of the UV source and the distancebetween the source and the target)

254 nm dose ranges for 99.9% (3-log)

inactivation of various microorganisms:

Virus: 2 – 16 mJ /cm²

Yeasts: 10 – 25 mJ /cm²

Bacteria: 4 – 60 mJ /cm²

Fungi: 15 – 400 mJ /cm²

Achieving Disinfection with UV

cell nucleus

cell wall

DNA

cell wall

cell nucleus

DNA

cell wall+pigments

DNA

bacteria

yeast

fungi

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VC

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UVC susceptibility of different microorganism classes

Fundamental Rules of UV Disinfection:1. UV disinfection is strictly a line-of-sight phenomenon – only that

which is exposed will be affected. There is no penetration into most materials.• This means shaded areas, crevices, and heavily textured objects will be

difficult or impossible to disinfect with UV

2. UV disinfection is an instantaneous effect – it does not create lasting resistance to recontamination• As soon as the UV irradiation is ceased, microorganisms begin to repopulate

the area

3. Transparency (transmission of visible light) rarely meanstransmission of UVC wavelengths• Don’t expect anything to transmit UVC radiation, even if you can see through

it clearly. Window glass, eyeglass lenses, clear plastic films are almost all completely opaque to UVC wavelengths.

Safety• UV is harmful to skin and especially eyes

• Sunburn is the effect of UVA & UVB overexposure on our skin

• Photokeratitis (see also: snow-blindness and arc eye) is like a sunburn on our eyes (especially the cornea)

• UVC exposure of eyes is especially dangerous because it is not immediately noticeable and its painful effects happen many hours later – you’ll wake up in the middle of the night feeling like you have sand in your eyes

• Cumulative exposure can cause permanent eye damage, e.g. cataracts

• Safety glasses and clothing are adequate protection against UVC, but be careful that stray light doesn’t “sneak in” around the sides and top of the safety glasses

The problem with door handles: they come in a variety of shapes, most of them complex with numerous surfaces

Because of their complex shape, the only viablemethod to UV-disinfect most door handles requires a handheld/portable lamp (i.e.a “wand”) to be moved around the handle in order to irradiate all surfaces.

Relying on human technique usually means inconsistent and non-uniform disinfection and extra expense for the labor. Robots can guarantee consistency and uniformity, but they are very expensive and not always compatible with the occupied environment.

Some other “halo” ideas using either a ring-lamp or a circular array of UVC-LED’s• Ring lamp is fragile and also requires warm-up time.

Probably not practicable in most situations.• UVC-LED’s are more durable and don’t require warmup time,

but would be quite expensive• Overall, this idea is complicated by needing a power source

on the door at all times and a control module to make sure that the lamp stays off when people are in the vicinity (for safety). This means cost & complexity.

Commercially viable applications using UV disinfection for water, air, and surfaces

My thoughts on UV disinfection and COVID-19*Note: I am not an epidemiologist, medical doctor, or even a biologist. These are my educated opinions as an engineer with modest knowledge of disinfection and microorganisms and based on what I have heard from various health professionals

Too much emphasis on surface disinfection! (Yes, I said it.)

• A mountain of research and case-study evidence shows that SARS-CoV-2 is primarily transmitted through airborne droplets, not fomites (surface transmission)

• Therefore, budgets and time being limited, efforts and money would be better spent on air disinfection instead of obsessive cleaning of surfaces

• Unlike UV surface disinfection, which must be done in unoccupied spaces, a UV air treatment system could be operating continuously while a building is occupied.

Thank you for your attention!

Feel free to contact me by email: [email protected]

Catskill-Delaware UV water treatment facility, the world’s largest UV installation:

• Max capacity of 2.24 billion gallons per day - treats 90% of NYC’s drinking water

• 56 UV reactors with 48” diameter pipes, each with 210 UV lamps = 11,760 total lamps