Nestlé India, maker of Maggi 2-Minute Noodles, is embroiled in a food-safety scare after inspectors filed a criminal complaint alleging the product contained dangerously high levels of lead.

The company said Monday that its own tests found the noodles safe to eat. That didn’t stop Delhi from banning the sale of the noodles on Wednesday saying the state’s tests found lead in some samples of the popular snack.

But how does lead, exposure to which causes an estimated 143,000 deaths a year worldwide, find its way into food?

Industrial emissions, car exhaust and even activities like shooting lead bullets at firing ranges can all put particles of the heavily-toxic metal in the air. For years, lead was added to gasoline until the U.S. banned it as an additive in 1986. By 2011 almost all countries, including India, had phased out leaded gasoline, according to the United Nations Environment Program.

It also gets into the soil and water through lead pipes and lead-based paints. The metal is a stubborn contaminant: it can travel far and it clings to soil and remains in the upper layers of dirt.

According to the United States’ Food and Drug Administration, once it is on food it “cannot always be completely removed by washing or other steps.”

Lead can find its way into everything from fruit juices and baby food to chili powder.

While it’s still unclear how the Maggi noodle samples might have allegedly been contaminated, experts say the way noodles are processed could have theoretically exposed them to lead.

Uday Annapure, an associate professor of the department of food engineering and technology at the Institute of Chemical Technology in Mumbai, said one source of contamination he could imagine was the water used to manufacture the noodles.

While none of the many food additives used in making the wheat noodles support lead, some components of the soup-flavoring packet, such as onion powder and wheat flour, “come from agricultural sources, all vulnerable to lead contamination,” Mr. Annapure said.

Even if the noodles and flavoring were clean, sometimes food can be contaminated by the packaging used. The United States Consumer Product Safety Commission said wrappers were responsible for unsafe levels of lead found in candy produced in Mexico in 2004.

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