The UV 254nm water quality parameter is related to the quantity of matter in the water which absorbs and scatters the UV light as it passes through the water.

The main type of matter that absorbs and scatters UV light is organic matter, which is very important in the water quality industry. By testing UV 254nm of a water sample, the level of organics in the water sample can be extrapolated.

There are of course many different types of organics found in water. The organics can be classified into two main categories – aromatic and aliphatic. Both categories of organic matter are commonly found in water. The structure of aliphatic organics is usually defined by long molecular chains of carbon atoms, whereas aromatic organics are usually ringed structures.

The molecular bonds between carbon atoms in aromatic organic molecules tend to be double carbon bonds whereas aliphatic organic molecules form mostly single carbon bonds. The double carbon bonds combined with a ringed structure causes aromatic organic molecules to have an abundance of "free electrons". The presence of these free electrons causes aromatic organics to have a higher degree of absorbtion of UV light than other organics..

The free electrons in aromatic organic molecules also tend to cause these ringed molecules to readily react with neighbouring molecules. This gives aromatic molecules a tendency to be more reactive than other organics. In fact, the aromaticity of organic molecules is often expressed as an indication of reactivity.

The reactivity of aromatic organics causes them to react particularly well with chlorine to form various other molecules. For this reason, when chlorine is added to water as a disinfection method, the organics already present in the water tend to react with the chlorine. Recently, it was determined that the disinfection by-products (DBPs) produced by the reaction between the aromatic organics and the chlorine are carcinogenic for humans. Because of this, great emphasis is now placed on monitoring and reducing the level of organics in the water before the chlorine is added. Since the UV 254nm water quality parameter measures the aromatic organic molecules particularly well, this organic water quality parameter is now considered the parameter of choice for this application.