Chlorpyrifos, a pesticide widely used in New Zealand but blamed for harming brain development in babies, is set to be banned in California after scientists deemed it to be more dangerous than previously thought.

California Environmental Secretary Jared Blumenfeld says it's the first time California, the most productive agricultural state in the US, has sought to ban a pesticide.

"This pesticide is a neurotoxin, and it was first put on the market in 1965," Blumenfeld says. "So it's been on the shelf a long time, and it's past its sell-by date."

Chlorpyrifos is used on about 60 different crops in California, including grapes, almonds and oranges. Hawaii banned it last year, and New York lawmakers recently sent a measure to the governor outlawing use of the pesticide.

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DowDuPont, which produces the pesticide, says it is disappointed with the decision and that it will hurt farmers' ability to control insects.

"It's a very important part of the crop protection tool box," said California Citrus Mutual president Casey Creamer. "We're fighting for our lives here trying to protect ourselves from deadly diseases, and we keep losing tools."

Creamer questioned the scientific studies behind the decision and said removing the pesticide could hurt efforts to prevent a pest like the Asian citrus psyllid from damaging the citrus industry in California like it did in much of Florida.

Blumenfeld said California took action in part because the federal government allowed the pesticide to be used after the Obama administration tried to phase it out.

The US Environmental Protection Agency under US President Donald Trump reversed an Obama administration decision to phase out the controversial pesticide but environmental groups and farmworkers challenged the reversal.

"This is a historic victory for California's agricultural communities and for children nationwide," says Miriam Rotkin-Ellman of the Natural Resources Defence Council. "The science clearly shows that chlorpyrifos is too dangerous to use in our fields. Since California uses more chlorpyrifos than any other state, this ban will not only protect kids who live here, but kids who eat the fruit and veges grown here."

The pesticide is in a class of organophosphates chemically similar to a nerve gas developed by Nazi Germany before World War II. Its heavy use has often left traces in drinking water sources. A 2012 study by the University of California, Berkeley, found that 87per cent of umbilical-cord blood samples tested from newborn babies contained detectable levels of the pesticide.

Dr Gina Solomon, a medical professor at the University of California in San Francisco, says chlorpyrifos is unusual in that it's one of the best understood pesticides because it's been so extensively studied. "We know a lot about what it does to developing children ... Many pesticides have been studied well in lab rats, but in this case, we actually know what it does to people."

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Studies in cities where the pesticide was once used to kill cockroaches before it was banned for indoor use in 2000 and in rural farmworker communities showed it harmed brain development in fetuses and affected reading ability, IQ and led to hyperactivity in children, Solomon says. Head sizes were smaller in children whose mothers were exposed to the pesticide.

The ban could take up to two years to take effect but the state Department of Pesticide Regulation has recommended adopting stricter rules on where and how the chemical can be applied.

To help farmers transition away from chlorpyrifos, California is adding contributing $9million to develop safer alternatives.

- AP