At its worst, our chemonoia can encourage us to give up otherwise healthy, potentially life-saving behaviours. Consider the fear of ingesting mercury in seafood. At high levels, mercury is a neurotoxin that can damage the brain, but the small amounts found in most fish are not enough to cause concern. “But because of the excessive fear, many people have given up eating seafood altogether,” says Ropeik. In doing so, they are missing out on key nutrients that would be important for brain growth and repair, and heart function: they are actually hurting their bodies more than if they’d enjoyed a plate of salmon. Even more worryingly, some parents are so scared of the small portions of mercury and other metals in common vaccines that they do not allow their children to be immunised – leaving them at risk of dangerous diseases like measles. (Despite fears still being espoused by the likes of Donald Trump and Jim Carrey, there is no evidence that the mercury or other metals found in vaccines can lead to conditions such as autism – whereas there is overwhelming evidence that immunisation saves lives.)

What can be done? Toxicologists should make a greater effort to openly discuss their findings, while recognising the very reasonable fears some people may feel – and journalists should make a greater effort to report them honestly, he says. But we should be careful about the ways we try to correct the misunderstandings. “Dismissing it as irrational makes people defensive and less likely to do something about it,” he says. “We need to accept this fact about ourselves, so that we can realise the ways that this is a part of our thinking and might be dangerous for us. Then we can do something about it.”

In some ways, it’s not so different from the monsters that once invaded our bedrooms as children. We may feel wiser as adults, but we are still scared of unknown, invisible, uncontrollable dangers lurking out of sight. And it is only through knowledge, reason and empathy that we can master those fears.

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This article has been updated to better reflect David Ropeik's background.

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David Robson is BBC Future’s feature writer. He is @d_a_robson on twitter.

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