What dose of UVC exposure is not harmful for human?

It is certainly interesting to know how dangerous UVC radiation is, what dose is acceptable for the tissue not to be damaged, and how much dose is needed for complete disinfection of microorganisms. Some math and theory, but most of all some practice.

We can therefore calculate UV dose on the following formula:

UV Dose = UV Intensity (μW/cm² ) x Exposure Time (seconds)

Exposure time is how long the UV light is ON and delivering UV to a surface. Start with 20 or 30 seconds. The result of this calculation will be a value expressed in  μWSec/cm².

Maximum dose which is not harmful to human is around 0,1 μWsec/cm2 or 0.1 μJ/cm2.

Viruses are mainly killed with dose from 2.000 μWsec/cm2 to 50.000 μWsec/cm2and recommended dose for UVC log(4) reduction of all microbes is at least 40.000 μWsec/cm2.

Limit values of UVC exposure to human without any indices

Limit valuesDuration of Exposure per day Effective Irradiance
Eeff (μW/cm2)
8 hours0,1
4 hours0,2
2 hours0,4
1 hour0,8
30 minutes1,7
15 minutes3,3
10 minutes5
5 minutes10
1 minutes50
30 seconds100
10 seconds300
1 second3.000
0.5 second6.000
0.1 second30.000
Source: “Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices”, the American Conference of Governmental Industrial Hygienists (ACGIH), 1991

But..

There is always a “but”. UVC radiation drops by square with distance.

The reduction in intensity, or power, drops very quickly. The simple formula for how fast this energy drops is power divided by the resultant quantity of distance away from the source raised to the second power (Ed = P/(d^2). Take a look at the graph below.

Starting with a “100%” energy value, (the energy measured or defined at point of data 1) is, incredibly, reduced to “25%” of the original energy at double distance! At three times of original distance, this power drops to around 10%.

The key point is that distance really matters. The best results of UVC energy happen in fairly close proximity to the target object(s).

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