Ultraviolet light is an electromagnetic wave. 254 nanometer wavelength light (253.7, more specifically) has inherent energy, energy that has an impact on what it touches. Just like a wave of sound, getting more and more quiet the farther away from the source you are, the same thing happens with UVC light. 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 above.
Every UVC system has to deal with this physics reality.
Starting with a "100%" energy value, the energy at four feet is, incredibly, reduced to "6.3%" of the original energy! At ten feet, the energy is only 1% of the initial energy. The key point is that distance matters. The best results of UVC energy happen in fairly close proximity to the target object(s). So, key point one: "Get the dose close."