A.D.H.D. is attention deficit hyperactivity disorder. Government estimates say three to seven percent of school-age children in the United States have it. Adults can also have it. People with A.D.H.D. have problems with paying attention, controlling their behavior and being overly active.

Now, a study has linked an increased risk of A.D.H.D. in children to exposure to some kinds of pesticides. The chemicals are known as organophosphate pesticides. They kill insects by attacking their nervous systems. Earlier studies have shown that some of the compounds cause hyperactivity and thinking deficits in laboratory animals.

Researchers at the University of Montreal in Canada and Harvard University in Massachusetts did the study. It involved more than one thousand one hundred children between the ages of eight and fifteen. One hundred nineteen of them were identified as having A.D.H.D.

The researchers tested all the children for products that break down from organophosphates. Maryse Bouchard of the University of Montreal and Harvard lead the research team. She says, "The higher the level of pesticide residue in the urine, the higher the risk of A.D.H.D. in the children."

The scientists reported that the risk of the disorder was almost double in children with high levels compared to those with no measurable level. They say exposure at levels common among children in the United States may increase the chances that a child will be diagnosed with A.D.H.D.

Maryse Bouchard says the study found that exposure could play a part in behaviors such as inattention and hyperactivity. But she also says the study does not prove that the chemicals cause the disorder.

MARYSE BOUCHARD: "We can¡¯t affirm that exposure to pesticides is causing A.D.H.D. We would need a different type of study to say that."

The researchers say forty organophosphate pesticides are registered with the United States government. The chemicals are used to protect fruits and vegetables. They are also used to poison insects in other places such as homes.

Lynn Goldman is an environmental health scientist at Johns Hopkins University School of Public Health in Maryland. She notes that the use of pesticides, including organophosphates, is widespread in developing countries. Children come in contact with high levels of pesticides through farming.

LYNN GOLDMAN: "And those kids have much more serious, much more severe neurological problems. So we do see evidence of effects in those populations globally."

The study was published in the journal Pediatrics.

A pacemaker is a small device that doctors place in people with an abnormal heartbeat. If a heart beats too slowly, the pacemaker will use electrical signals to help set a normal rate. Some devices include a defibrillator, which gives a shock if the heart beats too fast.

Pacemakers may be permanent or temporary. But one thing is sure. Developing countries need more of them as more people get heart disease.

A big problem, however, is the cost. Buying and implanting a pacemaker costs from five thousand to fifteen thousand dollars.

But doctors at the University of Michigan think they know a way to lower that cost. The idea is to reuse pacemakers.

Heart doctor Timir Baman estimates that more than one million people worldwide need pacemakers each year. He says reusing a pacemaker is an ethical way to provide health care to those who have no other way to get one.

TIMIR BAMAN: "A country such as Bangladesh or India, they average less than eight new implants per million. In the United States, we average seven hundred fifty-two new implants per million."

He got the idea a few years ago. One of his patients asked if someone might be given her pacemaker for reuse after she died.

But are used pacemakers safe? Doctor Baman studied medical reports about the safety of pacemakers that were being reused in small studies.

TIMIR BAMAN: "We found that there's no real difference in device infection or device malfunction when you compare it to new pacemaker implantation."

Funeral directors normally remove pacemakers when preparing bodies for cremation. Pacemakers can explode if they are burned. So Doctor Baman asked funeral directors in Michigan to send the pacemakers to him.

He and other researchers at the University of Michigan Medical Center tested the used pacemakers. They cleaned and disinfected the ones in good working order. Then they sent them to doctors in the Philippines, Vietnam and Ghana.

The doctors successfully implanted the used pacemakers in twelve patients. The findings were recently presented at a conference in Washington of the American Heart Association.

Now, Timir Baman has asked the United States Food and Drug Administration for approval to do a larger test. He says -- speaking by Skype from his office in Ann Arbor, Michigan -- that he is hopeful the program will work.

TIMIR BAMAN: "If we show that this is safe, other academic centers in the United States as well as in Europe can then form their own pacemaker reutilization programs and really help out countries in Africa, really help out countries in Asia, who really have no other access to these type of devices."

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