A new study has found that babies have a higher chance of dying in their first few weeks of life if their mothers live close to an oil spill.

Researchers studied information about babies dying and oil spills in the Niger Delta area of southern Nigeria. The researchers described their results as ‘shocking.'

The study comes from scientists from the University of Saint Gallen in Switzerland. They found that babies born in the Niger Delta were two times as likely to die in the first month of life if their mothers lived close to an oil spill before they became pregnant.

Nigeria is one of the largest oil producers in Africa. And, oil spills are common there. An estimated 240,000 barrels of unprocessed -- or crude -- petroleum oil are spilled into the Niger Delta every year. The environmental effects are clear to see: waterways covered with black liquid; animals that cannot breathe; dying trees and plants along the coast. Now, the effects on human health are becoming evident.

Roland Hodler was the lead researcher in the study.

"We looked at the birth histories of more than two-and-a-half thousand Nigerian mothers. And we compared siblings, some conceived before and some conceived after a nearby oil spill."

The researchers compared information on 6,600 recent oil spills with results from Nigeria's 2013 Demographic and Health Survey.

The study showed that even spills taking place five years before the women became pregnant doubled the chances of their babies dying after birth. However, spills that happened during pregnancy appeared to have little effect.

Hodler explains.

"We think the main reason is that some of the negative health effects are just building up over time. So, if you think about these negative health effects, these are due to skin contact with crude oil, or to drinking contaminated water or eating contaminated fish or crops. And also inhaling smoke from fires."

Unborn and newborn babies are thought to be at a greater risk as their bodies have yet to build up natural defenses. But the researchers say more studies are needed to show how crude oil affects health.

Crude oil is a mix of complex hydrocarbons – substances found in oil, coal and gas. Some are harmful to human health. So, Holder says, a second line of research would be to examine what parts of crude oil are the most harmful for both adults and newborns.

The new study suggests that the effects of oil spills will continue for many years after they happen.

In 2015, oil company Royal Dutch Shell agreed to share the costs of cleaning up its oil spills in the Niger Delta. The United Nations says the clean-up work will likely take 30 years. Critics say the company has paid just a very small amount of the money it promised for the effort. Shell blames oil thieves for causing many of the spills.

The Nigerian government is the majority owner of the Shell Petroleum Development Company, which directs most of Royal Dutch Shell's operations in Nigeria.

The Nigerian government did not respond to requests for comment.

I'm Alice Bryant.

Henry Ridgwell reported this story for VOA News. Alice Bryant adapted his report for Learning English. George Grow was the editor.

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barrel – n. a large, round container often used to hold liquid

sibling – n. a brother or sister

conceive – v. to become pregnant

survey – n. an activity in which many people are asked a question or a series of questions in order to gather information about what most people do or think about something

contaminate – v. to make something dangerous, dirty, or impure by adding something harmful to it

inhale – v. to breathe in

thief – n. a person who steals something

respond – v. to say or write something as an answer to a question or request