The infrastructure ministry has called for stricter rules for marine fuel oil used by Dutch ships following an investigation showing numerous instances of adulteration with harmful chemicals.

Fuel oil, the tar-like substance left after the refining process, is often mixed with chemicals to make is suitable for use in shipping. But these chemicals are bad for health, the environment and machines, and their use contravenes international treaties, according to transport ministry inspectors.

The preferred substance to make fuel oil suitable as a marine fuel is diesel but chemicals are cheaper and are seen by chemical companies as a good way of getting rid of waste products.

A survey among shipping companies showed the fuel clogged up filters and pumps and caused headaches and illness in crew members. A two-year investigation by Veritas Petroleum Service on behalf of the ministry concluded that 15% of the 3,6000 samples taken from Dutch ships were found to contain substances thought to have come from chemical companies.

In 2012, police estimated that almost all fuel oil was mixed with dangerous chemicals and  called for a legal framework for its use.

Some 10 years later, a police spokesman told investigative platform Follow the Money that the practice of adding waste chemicals “is standing practice and a great money-making scheme for environmental criminals”.

The latest investigation has confirmed the practice is still ongoing and stricter norms will be needed to bring it under control. The inspectorate also called on the shipping companies to “take their responsibility and prevent harmful chemicals from being mixed with fuel oil”.

According to infrastructure minister Barry Madlener, it will not be easy to track the composition and the origin of fuel oil as it is produced worldwide and on a large scale.

“Ships take in fuel everywhere in the world,” Madener said in a briefing to MPs. A further complication is that ships weigh anchor at different ports in countries where rules may differ, he said.