Major oil companies aren't declaring a significant source of greenhouse gas emissions according to a report.
The gas flaring at the oil fields where the companies work is not reported to the government.
Excess gas is burned during oil production.
The companies said their method was standard.
Flared gases cause pollution of the air and accelerate global warming.
High levels of cancer-causing chemicals have been found in Iraqi communities near oil fields. The fields have some of the highest levels of flaring in the world.
The UN special rapporteur on human rights and the environment compared these communities to modern sacrifice zones where profit and private interests are prioritised over human health and the environment.
The impact of the oil giants' toxic air pollution on children and the planet is revealed in an Arabic investigation.
The documentary is available to watch on the iPlayer in the UK, and will be broadcasted on the World News on Saturday 1 October at 07:30.
The need to eliminate all but emergency flaring has been recognized by companies for a long time.
The World Bank pledged in 2015 to end routine flaring by the year 2030.
It is the other company's responsibility to declare flaring emissions if they have contracted with another company to run day-to-day operations.
Half of these five companies' portfolios are made up of such fields.
Dozens of oil fields where these operators are not declaring the emissions is where no-one is.
We were able to identify the emissions from each site using World Bank data. Almost 20 million tonnes of CO2 equivalent went undiscovered from these flares. In a year, that's equal to the greenhouse gas emissions of 4.2 million cars.
All five firms said that the approach to reporting emissions was standard industry practice.
The World Bank pledge to cut emissions is not included in the emissions figure given by Shell and Eni.
People living near oil fields in Iraq have a higher risk of cancer, according to an investigation by the British Broadcasting Corporation.
People living in some of the world's largest oil fields in southern Iraq have long suspected that childhood leukaemia is on the rise.
According to a leaked Iraq Health Ministry report, new cases of all types of cancer in the Basra region have risen by 20% over the last three years. Air pollution is blamed.
The lead contractors at the oil fields are not the operators, so they don't report emissions. Operators do not.
The four sites were tested for cancer-causing chemicals over two weeks.
At least four places exceeded Iraq's national limit for benzene in the air tests.
We collected urine samples from 52 children and found 70% had elevated levels of 2-Naphthol.
The children have high levels that are concerning for their health and should be monitored closely.
Fatima was diagnosed with leukaemia when she was 11 years old. People who are exposed to benzene are more likely to develop this condition.
The Zubair oil field is where Fatima lived with her family.
There are no flaring emissions declared by the companies that operate there.
Iraqi law forbids flaring within a certain distance of people's homes.
The family's front door is 1.6 miles away from the flares.
Fatima drew the flames that surrounded her home.
She said she liked watching them at night.
Being on fire without being able to extinguish it was what her father experienced when she got sick.
Fatima died as her family tried to get a transplant. She was 13 years old
The company said it "strongly rejects any allegation that its activities are endangering the health of the Iraqi people".
According to the company, it does not have responsibility for flaring in the area.
The oil field is 25 miles away and it flares more gas than any other site in the world.
The operator of the Rumaila Operating Organisation is supervised by BP. Flare up from the oil field is not declared by either party.
Those who are affected by pollution levels that exceed national limits are legally entitled to compensation.
Ali Hussein Julood, a 19-year-old leukaemia survivor, said that he and his father were met with silence when they tried to get compensation from the oil company.
"We are very concerned by the issues raised by the BBC, and we will review those concerns immediately."
Iraq's Oil Minister told us that they instructed all the contracted companies to uphold international standards.
According to the International Energy Agency, if all the natural gas was captured and used, it could replace more than a tenth of Europe's gas imports from Russia.
The World Bank says it can be hard to capture the gas. It estimates the cost to end all routine flaring to be as much as $100 billion.
Mark Davis, chief executive of Capterio, which advises oil companies on capturing flared gas, said that countries like Norway have shown it can be done.
The additional support came from Christine Jeavans andBecky Dale.