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Dose of poison

Posted 15/05/2012 by sevans

According to Paracelsus, ‘The dose makes the poison’; in other words, what matters is not so much the substance but how much of it is present. In modern usage, international regulators and scientific authorities have interpreted this by assigning to different chemicals so-called threshold values related to their observed or projected toxicities, below which doses we are led to believe they may be safely used and applied. LD50 values, at which concentration of substance half of the cells in a population will die, are another measure adopted to instil confidence that we have some handle on safety.

But can we be sure that staying below the recommended levels will keep us safe from adverse effects? With so many chemicals now circulating in the environment, is it wise to simply assume we are safe below a pre-assigned dose of any one of these without taking account of potential interactions with others?

The question of dosage is relevant to a story this week concerning endocrine disruptors, chemicals such as phthalates and bisphenol A in plastics that interfere with the hormone systems of animals, including humans, occasionally as in the case of certain fish and alligators with headline-making ‘gender bending’ consequences. While evidence of the effects of such chemicals at higher doses is well established, some chemicals industry representatives have long argued that the tiny amounts of these substances in specific products are too low to be of real concern.

In an interview this week with EurActiv, however, Shanna H. Swan, a scientist specialising in reproductive medicine, has called on regulators to better protect consumers from the health effects of endocrine disruptors even at tiny doses which may have particularly detrimental effects on developing foetuses. In a study measuring hormone levels of male rats in the womb, Swan claims to have detected tiny differences in testosterone depending on where the males were located between two males or between two females – proof she says that tiny doses of hormones can alter the development of foetuses.

And, as a recent paper by Tufts University researchers in Endocrine Reviews at the end of March 2012 highlighted, the effects of EDCs on the body may be non-linear and varied within a range of doses studied.

Elsewhere in the animal kingdom, other recent research on bees has also pointed to the dangers of trusting in threshold values, with the finding that bee migratory pathways may be compromised even in the presence of low sub-threshold levels of neonicotinoid insecticides.

Paracelus may have been right, but knowing exactly what dose makes the poison remains as contentious as ever.

Cath O’Driscoll - Deputy editor

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