Sweden started ringing the alarm bell more than 20 years ago, calling for a ban on the weedkilling chemical paraquat in 2004. One by one, dozens of other countries did the same as evidence of paraquat risks to human health mounted. Even the Chinese government, which owns the longtime paraquat maker Syngenta, halted domestic use of the pesticide a decade ago after conducting its own studies on paraquat safety and finding troubling results.