Looking back, it seems insane. Bluntly put, we took a known poison and – for three quarters of a century – used it in machines that puffed it out in breathable form. Then we drove them millions of miles a day, all over the world, regularly dosing billions of people with the toxin.

Now the full effects of using lead in petrol – surely the greatest ever mass poisoning experiment – are becoming clear, almost exactly 35 years after an award-winning Observer campaign caused it to be banned in Britain. They seem far worse than anyone imagined, and provide a stark warning against cutting pollution standards after Brexit. Last week, a massive new study concluded that lead is 10 times more dangerous than thought, and that past exposure now hastens one in every five US deaths. A similar effect is expected in Britain and other industrialised countries.

The study, published in the Lancet Public Health journal and believed to be the first to research the effects of low levels of lead exposure on the general public, also concludes there is no safe level of the toxic metal: people with the lowest detectable amounts were still affected.

Researchers at four North American universities, led by Bruce Lanphear, of Simon Fraser University in Vancouver, studied the fate of 14,289 people whose blood had been tested in an official US survey between 1988 and 1994. Four fifths of them had harboured levels of the toxic metal below what has, hitherto, been thought safe.

The study found that deaths, especially from cardiovascular disease, increased markedly with exposure, even at the lowest levels. It concluded that lead kills 412,000 people a year – accounting for 18% of all US mortality, not much less than the 483,000 who perish as a result of smoking.

Philip Landrigan, of New York’s Mount Sinai School of Medicine, possibly the world’s top expert on pollution’s effect on health, endorses the study and Alastair Hay, an expert on lead from Leeds University, calls it “very well done”. He adds that, since blood lead levels in the UK were similar, there could be much the same effect as in the US.

Of course, we suspected none of this when this newspaper campaigned against lead back in the early 1980s. We pressed for a ban because of increasing evidence – dismissed by the scientific establishment – that low levels of the toxic metal were damaging children’s brains, lowering IQ and increasing disruptive behaviour.

The danger of high lead levels has been known at least since the second century BC, when the Greek physician, Discorides, noted that it made “the mind give way”. Some historians even believe that lead-lined wine vats helped cause the decline of the Roman empire.

An especially nasty form of the metal, tetraethyl lead, was first put in petrol to prevent car engines “knocking” in 1922. By the early 1970s, more than 100,000 tons of it were put into American vehicles alone each year. By then, a lone scientist, Derek Bryce-Smith, of Reading University, had begun to campaign against it. He would say that he had first been alerted to the dangers when, as a young man, he had asked the manufacturers for a sample of tetraethyl lead for an experiment and was warned that “if I got any on my finger it would drive me mad or kill me”.

He was ridiculed and marginalised by other scientists. Slowly, however, people took up the cause, most notably a Wimbledon housewife, Jill Runnette, who ran a campaign that – against all the odds and entrenched scientific opinion – succeeded in persuading ministers to reduce its level in petrol by two thirds in 1981.

British children had to be shown to be affected as well as US ones – as if they were different species

Her hand was strengthened by a meticulous US study that showed conclusively that lead knocked several points off children’s IQ. Yet, even before it was published, four leading British authorities circulated a paper attacking it. And the study’s author, Herbert Needleman, was twice investigated for scientific misconduct. He was vindicated but ministers still refused to act on lead until a UK study had come up with the same result. One told me that British children had to be shown to be affected as well as US ones – as if they were different species. The final blow was struck by a year-long public campaign led by the activist, Des Wilson, and – in April 1983 – the government finally announced that the use of the toxic metal would be phased out. Crucially, it did so on a precautionary basis.

With leading scientists still resisting the evidence of effects on children, a royal commission advised that action should be taken, short of absolute proof, as there were enough grounds to suggest the danger was real. First, the rest of Europe, then almost every nation on Earth, followed, and levels in blood have fallen worldwide.

It was not before time. Since then, low levels of lead have also been linked to high blood pressure, stroke, heart and kidney disease, ADHD and accelerated ageing. There is even good, if surprising, evidence that it increased juvenile crime: many studies have shown rates mirroring the rise and fall of lead in petrol. In 2002, the World Health Organisation called lead in petrol “a catastrophe for public health”.

Worryingly, the “precautionary principle” that led to the phase-out, and is enshrined in EU law, is in danger from Brexit. Some industries and rightwing politicians have long campaigned against it, demanding much more evidence. So far at least, it has not been incorporated into withdrawal measures. On Thursday, the environment secretary, Michael Gove, promised at a conference organised by Prosperity UK to strengthen, rather than weaken, pollution controls on leaving the EU. But he failed to give a specific reassurance on the precautionary principle.

This is important because undue delay kills. Absolute proof of damage comes from counting corpses. If Britain had waited for it before taking action on lead in petrol, this week’s alarming news would have been more devastating still.