(CNN) It's a mantra repeated by doctors all over the world: There is no safe amount of lead.
Lead exposure can adversely affect nearly every system in the body. And because it often occurs with no obvious symptoms, it frequently goes undiagnosed until it's too late. Children younger than age 6 are especially vulnerable to lead poisoning, which can severely affect mental and physical development.
"When pediatricians hear anything about lead, we absolutely freak out," said Dr. Mona Hanna-Attisha, the Flint physician who sounded the alarm on the city's water crisis. "Lead is (an) irreversible, potent neurotoxin."
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Initially, lead poisoning can be hard to detect. Even kids who seem healthy can have high blood levels of lead. Making matters worse, signs and symptoms might not appear until weeks, months or even years after exposure.
Parents should watch for developmental delays, learning difficulties, irritability, loss of appetite, weight loss, sluggishness and fatigue, abdominal pain, vomiting, constipation and hearing loss, and have children tested if they suspect they've been exposed to lead.
Most doctors recommend children's lead levels be tested during routine well exams. The test is covered by Medicaid and most private health insurance. At minimum, the American Academy of Pediatrics recommends children be tested at ages 1 and 2.
The blood test is simple, and levels are measured in micrograms per deciliter. A level of 5 mcg/dL or higher indicates a child might have unsafe levels of lead in his or her body and should undergo further testing.
If levels exceed 45 mcg/dL, doctors will initiate treatment for lead poisoning. The most common treatment for severe cases, known as chelation therapy, involves taking a medication that binds with the lead so that it's excreted in urine.
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"We care about (lead) so much because it impacts your cognition and your behavior," said Hanna-Attisha. "It actually drops your IQ. Imagine what we've done to an entire population. We've shifted that IQ curve down. We've lost our high achievers, the next kid who's going to be a neurosurgeon, and we have all these children who may now need remedial services."
Hanna-Attisha said lead poisoning has also been linked to ADHD, impulsivity and even criminality.
At very high levels, lead poisoning can be fatal.
Lead affects everyone differently, though in almost all cases, every single organ system is impacted. The damage is lifelong and irreversible.
"In five years, when kids are diagnosed with a learning disability, we can never say it was from the lead, or if that kid was always supposed to have a brain disability," said Hanna-Attisha, adding that "it increases the likelihood of all these things happening."
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At least 4 million households have children being exposed to high levels of lead, according to the Centers for Disease Control and Prevention. There are approximately a half-million U.S. children ages 1 to 5 with blood lead levels above 5 mcg/dL, the level at which the CDC recommends medical intervention.
The most important thing you can do at home is to prevent lead exposure before it occurs.
Although there are several exposure sources -- air, food, water, dust and soil, among them -- lead-based paint is the most widespread and dangerous high-dose source of lead exposure for young children. Lead is invisible to the naked eye and has no smell. The CDC stresses that children are exposed to lead from different sources, such as paint, gasoline and consumer products.
"A lot of what we do in pediatrics is reassurance," said Hanna-Attisha. "We try to reassure (parents) that not every kid is going to have every problem, but we also try to give them practical things that they can do now. There is no pill for lead. There is no antidote for lead. But there are things we can do to mitigate this exposure."
Nutrition, for one, plays a huge role. Some foods can help limit the absorption of lead: foods high in calcium, such as milk, yogurt, cheese and green leafy vegetables; iron, including lean red meats, beans, peanut butter and cereals; and Vitamin C, such as oranges, green and red peppers and juice.
"When (lead) gets into your body ... it can get excreted or it can get absorbed in your bones. That's where calcium plays a role. If you are fully loaded with these vitamins, it will limit the absorption of lead into your body."
Avoiding an empty stomach helps too, said Hanna-Attisha.
"You absorb more lead if you have an empty stomach. ... It's important for kids to have fuller stomachs -- so, lots of snacks."
And plenty of mental stimulation to help those developing brains can help mitigate any possible problems, Hanna-Attisha said.
"Read to your kid, talk to them, stimulate them," she said. "Put them in early literature programs, enroll them in preschool."