CONTAMINATED LAND AND THE ASSESSMENT OF RISK

A tale of toxicity and jay-walking chickens !

One sometimes finds that a brownfield site may be described as ‘toxic’ and that this alleged “toxicity” is put forward as a reason for inhibiting its redevelopment.  As is often the case, the populist use of the word has little in common with its meaning in science.  This article discussing the matter and, more importantly, explains why the chicken crossed the road.

Some alarmism
Those who correspond on sites which they headline as being ‘toxic’ often provide a list of the portentous sounding chemicals which the site is said to contain or emit.  These lists need to be taken with a pinch of sodium chloride because an inscrutable chemical label is no indicator of a potential for harm.

For example, the hazard assessment for a particular mixture lists ingredients “presenting a physico-chemical, health or environmental hazard within the meaning of the CHIP Regulations or which are assigned occupational exposure limits” and they included naphtha (petroleum), calcium carboxylate, ethyl methyl ketoxime. If it was said that these chemicals were found on a site, then there might be cries of alarm.  Certainly, one would not wish to have this substance in his house, which is unfortunate because it is a well-known brand of gloss paint ! 

Then consider diethanolamine, sodium hydroxide, diethylene glycol monobutyl ether or monoethanolamine.  Again, this would sound worrisome if found on a site. In fact, these ingredients can be found in many oven cleaners.

The reporter may cite the mere presence of radioactive materials as his bull point; however, it is worth remembering the self-luminous paint on those watch and clock dials which glow because of radio luminesce.  The paint that consists of a small amount of a radioactive isotope which continually decays, emitting radiation particles as it does so.

The horror continues when a householder is told that his house is being bombarded with ‘electro-magnetic’ radiation.  He might, or might not, be relieved when he is told that this is, in fact, sunlight.  Sunlight is, of course, safe  . . .  is it not ?  This is right, at least up until the point it causes skin cancer.  This gruesome example forms a neat segue into the principle point of this essay; namely, that it is a matter of dosage.  A little sunshine is a good thing, but too much can kill you !

A disagreement with some lexicographers ?

Those who are interested in such things will know that the word ‘toxic’ was named as the Oxford Dictionaries word of the year in 2018.  For the purposes of this article, the dictionary defines “toxic” as “poisonous” with its roots derived from the medieval Latin term “toxicus,” meaning poisoned or imbued with poison.  The version of the Oxford Reference Dictionary which sits on my bookshelf defines it as “of, caused by, or acting as a poison”.  The book also says that the word is derived from the Greek “toxa”, which means arrows. [1]  The origins of the term are best left to the philologists; however, neither definition is particularly accurate.  The Merriam-Webster dictionary can be criticised for the same where it defines the word as meaning “containing or being poisonous material especially when capable of causing death or serious debilitation”. 

Not surprisingly, the phrase ‘non-toxic’ is defined as meaning not “poisonous or not containing poisonous substances” in the Cambridge English Dictionary.  The Collins English Dictionary gives “not of, relating to, or caused by a toxin or poison”.

Thus, ‘toxic’ is equated with ‘unsafe’ and ‘non-toxic’ is equated with ‘safe’.  Unfortunately, a toxicologist would not approach the matter in this way.

A sense of proportion

‘Toxicity’ is the degree to which a chemical substance (or a particular mixture of substances) can damage an organism, and this is dose-dependent.  For example, even water can lead to water intoxication when taken in too high a dose, whereas for a very dangerous substance (such as snake venom) there is a dose below which there is no detectable toxic effect.  Paracelsus is said to have expressed the classic toxicology maxim "All things are poison, and nothing is without poison, the dosage alone makes it so a thing is not a poison.  This is why public health standards specify maximum acceptable concentrations of various compounds in food and drink rather than seeking to impose unrealistic and unnecessary absolute embargos on the use of these compounds.

Also, as we have learned from the coronavirus pandemic, toxicity varies from subject to subject. [2]  Some people can absorb more of a virus or chemical than others.  Typically, children are more vulnerable to potentially debilitating chemicals than adults.[3]  And big young adults are usually less vulnerable than smaller older adults.  Accordingly, those agencies which specify acceptable safe limits of absorption differentiate between different classes of subject.

Landfill gas is often given as an example of a dangerous substance.  In high concentrations it is an asphyxiant and explosive.  However the real life incidents of actual harm and are few and far between.  Most textbooks on the subject refer to an explosion which destroyed a house in Loscoe, Derbyshire, but it is usually the only example they give.  This is because such incidents are a rarity.

The situation is different with domestic gas supplies.  Like landfill gas, it can be explosive and an asphyxiant in high concentrations.  Given that the principal constituent of both landfill gas and domestic gas is methane, the similarities are to be expected.  Yet millions of households willingly accept the supply of domestic gas for the purposes of heating and cooking.  This is notwithstanding that (unlike landfill gas) there are many reported incidents of domestic gas explosions every year.  Then, of course, there is the ever present danger of the carbon monoxide which results from the degradation of unburned domestic gas.  This is despite the fact that gas appliances, and their installation, must be in accordance with very high safety standards.  If one examines the situation objectively, then it must be the case that those who set and monitor those safety standards do so knowing that the importation of gas into the home brings with it an ever present risk of harm.  It must be the case that this risk of harm is deemed to be acceptable.  These dangers can be mitigated by way of free flowing ventilation; however, the principal means of room ventilation is by the windows which are, traditionally, closed at night.  It is possible to install gas boilers in self-contained and well ventilated enclosures, but this is not something one sees in modern housing estates.  One answer might be to use electricity instead of gas.  Unfortunately, the importation of electricity into the home brings with it its own dangers.

The reality is that, so far as they consider the point at all, householders are willing to accept the benefits of domestic gas and electricity supplies notwithstanding any attendant dangers.  Likewise, those who are responsible for setting safety standards must do so on the basis that the rewards exceed the risks.

It is right to say that landfill gas has the potential for harm, but one has to put this into perspective. The question is not whether it is harmful per se, but whether the risk is outweighed by the reward.  Where the risk of harm for landfill gas on a brownfield site is low, then this might be outweighed by the benefits of its development.  

Those involved in the world of statistics are fond of pointing out that all activities involve an element of risk.  Ergo, the commonly cited example of crossing the road to get to the other side.  The chicken crossed the road because the reward from doing so outweighed the risks. Albeit of course, neither poultry nor humans normally consciously carry out a risk assessment before doing so.  It is commonly pointed out that the risks of harm by flying are much less than those involved in driving a motor car. Notwithstanding, many people perceive flying to be an extremely hazardous activity.  The perception of risk and its reality are not always the same thing.  It therefore follows that decisions involving the development of contaminated land should, ideally, be based upon science rather than subjective perception or populist belief.







[1]     En passant, the book also says that a ‘toxophilite’ is a student or lover or archery. 
[2]     I have the dubious distinction of falling into the ‘vulnerable’ category due to the fact that I am 70. 
[3]     Ironically, whilst it underpins the point about different doses for different people, it appears that children are less vulnerable to Covid 19 than adults.

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