Limits of detection

Why is it that water, food or drink that was described as pesticide or contamination free before is now being said to be contaminated?

Sometimes it is because of the limit of detection. The limit of detection (LOD) is a specific term used in analytical chemistry. It defines the limit of the test in determining absence from being 99% confident that the substance being tested for is present. As tests and analytical equipment is developed and improved this LOD becomes lower and lower. This means that on one occasion a pesticide or chemical contaminant may be deemed as ND - not detected, but that at a future point in time as the degree of accuracy improves what was deemed as not detected is now seen as a positive result.

To what level can we detect? well down to microgrammes or nanogrammes. How small is this? Well we would be familiar with a kilogramme bag of sugar and how much it weighs in our hand. This is one thousand grammes let me explain a different way.

1 Tonne (metric) = 1000 kilogrammes (kg) or 1000 000 grammes (g)

1 Kilogramme (kg) = 1000 grammes (g)

1 gramme (g) = 1000 milligrammes (mg) = 1000 000 microgrammes (ug) = 1000 000 000 nano grammes (ng) or 1000 000 000 000 picogrammes (pg)

Therefore the limit of detection can be at microgramme or nanogramme levels.

It can be more confusing still because contamination is often expressed in "parts per ...." This is an expression of proportion there is one part of this chemical compared to the total number of parts e.g. one part per million (ppm); one part per billion (ppb). As one litre of water weighs one kilogramme (kg), contamination can be expressed as one part per million (1 ppm) or one milligramme per litre (written in scientific shorthand as 1 mg/l); one part per billion (1 ug/l) - these are very small quantities.

Does the presence of chemical at this level cause illness - either acute (instant) or chronic (long term)? Well this is the ultimate question and thousands of scientists around the world are working on this at the moment. Policy makers have to make a formal decision now so they have to use current science to set safe limits for our food and water.

In the future what is deemed as safe now may through research be determined as unsafe; what is deemed as contamination free may then be deemed as contaminated. Science evolves continually. George Bernard Shaw is reputed to have said that "Science ... never solves a problem without creating ten more." How true ...

First published at The Human Imprint