Scientists at universities with no stakes in the biotech industry have also questioned and rigorously evaluated the risks of B. thuringiensis and its toxins ever since farmers started using Bt sprays in the 1920s. Numerous laboratory and field tests have concluded that Bt is not toxic to fish, birds, mammals or people, even at doses thousands of times greater than what a person or animal would ever encounter outside the lab. Over the years researchers have injected or piped billions of Bt spores and toxic crystals directly into the skin, lungs, blood, stomachs and brains of mice, rats, cows, pigs, hens and quails; time and again the animals survived the experiments with few, if any, ill effects. The same is true for rats that ate one billion Bt spores a day for two years as well as for three successive generations of rats fed Bt corn. Joel Siegel, now at the U.S. Department of Agriculture, spent more than 10 years investigating the toxicity of Bt. "My conclusion was this was a very safe product," he says. "You could probably eat a pound of it and nothing would happen to you.”
In the 1950s volunteers for an experiment that today's ethical committees would probably never approve did in fact eat Bt. Each day for five days 18 people ingested one gram of a Bt spray called Thuricide—containing approximately three billion Bacillus spores and crystals—and inhaled 100 milligrams of the insecticide. Detailed physical examinations, blood tests and x-rays on the sixth day and five weeks later revealed no unusual or harmful changes. Although no one has ever developed a serious illness or died from ingesting B. thuringiensis or Bt crops, research suggests that a small percentage of people routinely or accidentally exposed to plumes or splashes of commercial Bt sprays have suffered skin rashes and irritated eyes. When they work as intended, Bt crops eliminate this danger and reduce workers' exposure to pesticides in general. Bt crops also indirectly improve human health. More than half of corn grown worldwide is infected with fusarium fungi, which sneak into the plants via tunnels formed by boring insects and, once established, produce toxins that damage the kidney, liver, nerves and cardiovascular system if ingested in high doses. Bt crops that kill such insects have 90 percent fewer fungal toxins than conventional crops.
A small minority of anti-GM scientists say that a handful of worrisome studies counter the decades of research demonstrating that Bt is not poisonous to people. In each case the greater scientific community has thoroughly criticized and often outright rejected the supposedly alarming studies because they were flawed, invalid and sometimes masked ulterior motives.
Gilles-Eric Séralini of the University of Caen Lower Normandy in France has published several highly controversial studies purporting that GM plants cause tumors, kidney failure and other maladies in rodents, sometimes killing them, and that Bt toxins harm human cells. Numerous scientists and scientific organizations—including those with no ties to the biotech industry—have excoriated Séralini’s experiments, stressing their defects: they have generally lacked the necessary statistical power to rule out illness due to chance; some studies used short-lived, lab-bred rats that are prone to tumors; the experiments using naked cells in petri dishes in no way reflected how the human body comes into contact with Bt; and the studies were often vague on important details or excluded them altogether.