100 years ago, scientists discovered something revolutionary: if you exposed pathogens to ultraviolet (UV) light, it was difficult for those germs to reproduce.  They specifically discovered that light in the 254 nanometer (254 nm) range effectively inactivated germs and pathogens.  (All ultraviolet light is calculated on a UV wavelength.)

As a result, today it is common for disinfection technology to incorporate ultraviolet light (specifically UV light in the UVC spectrum) in order to target, disable and effectively inactivate disease-causing microorganisms.  In the fight against the inactivation of diseases, ultraviolet disinfection technology plays an ever-increasing role.

A common question customers ask of us at LightRay UVC is how much contact time for UV disinfection is required; essentially, how long does it take for a surface to be disinfected. “How long do my surfaces need to be exposed to UVC light?” This crucial question can require a significant amount of complicated math depending on the application (we lay this out in more detail below); essentially, however, the effectiveness of the exposure to germicidal ultraviolet light is the product of time and intensity. High intensities for a short period and low intensities for a long period are fundamentally equal in lethal action on bacteria. 

An additional crucial element however besides time and intensity when it comes to effective ultraviolet exposure is the proximity of the germicidal lamps to the subject. Distance from the lamps is also critical to defining the effectiveness of UVC lamps.  When it comes to germicidal ultraviolet light, the inverse square law equally applies: the killing power decreases as the distance from the UV lamps increases.

To speak in generalities (while it’s dangerous to make gross generalizations without all the data), scientists have found that the average bacterium can be killed in ten seconds at a distance of six inches from certain high-intensity UV lamps.  It is important to stress however that not all pathogens are created equal, and time and effectiveness will vary depending on both the germ and the intensity and distance of the germicidal lamp you choose to use. 

Water-Based Pathogens

Similarly to surface-based bacteria or pathogens, when water pathogens are exposed to UV light, their cells become damaged, which inhibits their reproduction, effectively inactivating them.  The UV light produced by special lamps damages the DNA and RNA of the cells and, once damaged, they are unable to replicate.  The exposure to UV light similarly renders water-based pathogens harmless. 

In the same way as surface-based UV lamps, the amount of damage to water-based germs and bacteria is a result of the intensity of the UVC output multiplied by the amount of time the water is exposed to UV light.  The applied dosage is commonly referred to as microwatts and is often expressed as mJ/cm2.  Doses of 40,000 microwatts (40 mJ) are accepted for water disinfection.

As a result – when it comes to evaluating successful UVC treatment, no matter the surface or substance, it’s not just enough to consider time and intensity of exposure to UV light; you want to make sure your germicidal lamps are proximate to the surface or substance in order to achieve maximum effectiveness. 

As a reminder, many health and safety workers are currently working in biosafety cabinets – enclosed, ventilated laboratory workspaces for safely working with materials contaminated with (or potentially contaminated with) pathogens that require a defined biosafety level. These cabinets are best cleaned and serviced by UV disinfection systems.

• UV cleaning systems reduce and even eliminate pathogens that can otherwise lead to hospital-acquired infections.
• Germicidal light fixtures increase efficiency, prolong HVAC equipment life and improve indoor air quality.
• UVC light fixtures inactivate mold spores, bacteria, viruses and other undesirable microscopic contaminants from food and beverage products and packaging.

All of these cleaning services and systems are provided by LightRay UVC, and are crucial in the fight against pathogens, particularly for those health and safety workers hardest hit and currently working in spaces that require constant cleaning and care.

For more information on how to calculate the most effective amount of exposure time to ultraviolet light, please continue reading below.

Calculating Exposure Time To Ultraviolet Light 

In order to find the time required for proper exposure of ultraviolet light to germs, you must first understand how to calculate the required UV dose to meet your goals when it comes to pathogen deactivation and disinfection.

Step 1: Calculate Dosage

For surface disinfection applications, the UV dose is the product of the irradiance and exposure time:

Using the above equation, you can find that the time necessary to disinfection a surface is equal to the quotient of the dose divided by the irradiance. Therefore, when considering how much time is required for surface disinfection, you must first determine how disinfection is defined (target pathogen and log reduction).

Step 2: Calculate Irradiance

The next step is to determine the irradiance, which is defined as the power per unit area incident upon a surface from a specified distance. This is inversely proportional to the source-to-surface distance (known as the inverse square law function). Once we have calculated the dose requirement and calculated irradiance, it’s fairly straightforward to use the above equation to determine the time necessary for the given disinfection target.

For instance, let’s consider a defined disinfection level of 99 percent reduction (i.e. 2 log reduction value) of staphylococcus aureus, and a target surface area of 10 cm x 10 cm.

Step 3: Calculate Time

In order to determine the time required, we will have to determine:

1. The wavelength-specific dose required;
2. The number of LEDs and relative placement/distance;
3. Minimum irradiance point on the surface.

• The dose required to achieve this level of disinfection is 5.4 mJ/cm² at 254 nm, which is an equivalent dose of 5.2 mJ/cm²at 265 nm or 6.7 mJ/cm² at 280 nm.
• We use a single Klaran 35R GD LED operated at 350 mA and at beginning-of-life, aligned to the surface center and placed 6 cm from the surface.
• The minimum irradiance value measured at the corner of the surface is 0.075 mW/cm², which is approximately 25 percent of the peak irradiance value measured at the middle of the surface. (This is modeled using an optical design software called Zemax, but can be estimated for a single LED, considering a point source and using the specified optical power and viewing angle.)
• With this information, we can calculate the minimum time required for the whole surface to be disinfected for a 2 log reduction value of staph. aureus: 

It is important to note that this time required is not absolute. It is impacted by the inputs on dose and irradiance (which is impacted by the UVC LED power, arrangement and distance to the surface). 

The key point to understand is that time and irradiance are correlated. Thus, if certain parameters of a system are open (e.g. the distance between the light source and the surface, the use of more LEDs, higher drive currents) engineers can play with these variables to minimize the exposure time, or inversely increase exposure time to lower operating characteristics.

(Our thanks to the good people at Klaran University for the mathematical equations.)

It should be noted that our professional team at LightRay UVC is well-educated and versed in these mathematical equations, making sure your exposure time for each room where a UV lamp is used is effectively inactivating and killing known viruses and pathogens. This is why it’s important to hire a professional. Whether you’re hiring our services or would like to purchase the equipment for constant use, we’re here to help during these trying times.