Three new studies suggest links between natural gas wells and cases of cancer, asthma and birth problems.

Children who lived closer to natural gas wells in heavily-drilled western Pennsylvania were more likely to develop lymphoma, a rare cancer, researchers found. Nearby residents of all ages had an increased chance of severe asthma, a lung disease, and low birth weight.

The University of Pittsburgh did the studies.

The researchers found what they called strong connections between the gas industry activity called fracking and two health problems: asthma, and lymphoma in children. Lymphoma is a type of cancer less common among children.

The researchers were unable to say whether the drilling caused the health problems. The studies were not designed to do that. Instead, the researchers looked at health records to try to find out possible connections based on how close people lived to natural gas wells.

Gas industry groups say the studies had weaknesses, and had limited data.

In the cancer study, researchers found that children who lived within 1.6 kilometers of a gas well had five to seven times the chance of developing lymphoma compared with children who lived 8 kilometers or farther from a well. That is equal to about 60 to 84 lymphoma cases per million children living near wells. For kids living farther away, the number is 12 per million.

The researchers found that people with asthma who lived near wells were more likely to have severe reactions. However, researchers said they found no clear connection for severe reactions during periods when crews were building, drilling and fracking the well. Fracking is the process of putting liquid into the ground to extract natural gas.

The four-year, $2.5 million research project is coming to an end. It was established under pressure from the families of childhood cancer patients who live in the nation's largest natural gas reservoir in western Pennsylvania. A very rare form of bone cancer had been found in dozens of children and young adults in a heavily drilled area outside the city of Pittsburgh.

But the researchers said they found no connection between gas drilling and childhood leukemia, brain and bone cancers.

The researchers said their findings on early births and low birth weights among families living closer to gas wells were similar to the mixed results of similar studies. But it said babies born to mothers near active wells in production were about 28 grams lighter on average.

Edward Ketyer is a retired child doctor who sat on a board for the study. He said he expected that the studies would be similar to other research showing the "closer you live to fracking activity, the increased risk you have of being sick with a variety of illnesses."

A number of states have strengthened laws around fracking and waste disposal over the past 10 years. However, researchers have said that it is still unclear how much toxic substances the industry puts into the air, injects into the ground or produces as waste.

Large-scale fracking the past 20 years has made the United States into a worldwide oil and gas superpower.

But it also brought many protests about water and air pollution, as well as diseases, in states including Texas, Colorado and Pennsylvania.

I'm Dan Novak.

Dan Novak adapted this story for VOA Learning English based on reporting by The Associated Press.

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drill — n. tool used for making holes in hard substances

resident — n. someone who lives in a particular place

extract — v. to remove by pulling it out or cutting it out

reservoir — n. an extra supply of something

dozen — v. a group of 12 people or things

variety — n. a number or collection of different things or people

illness — n. a condition of being unhealthy in your body or mind

disposal — n. the act of throwing something away

toxic — adj. containing poisonous substances