SANTIAGO (Reuters) - Chile’s police chief said on Tuesday he has suspended the use of rubber bullets to quell national unrest after a university study revealed they could be made up of as little as 20% rubber and contain harmful substances such as lead.

FILE PHOTO: A demonstrator shows a rubber bullet and its cartridge used by the riot police during a protest against Chile's government in Valparaiso, Chile November 12, 2019. REUTERS/Rodrigo Garrido/File Photo

Director General Mario Rozas said he had sought clarification from the manufacturer, and commissioned independent studies from within and outside of Chile, in a statement tweeted by police.

He said from now on, as a “prudent” measure the bullets could only be used, along with firearms, “as an extreme measure and exclusively for self-defense, when there is imminent danger of death.”

The intervention came after a study by the Universidad de Chile, commissioned by Santiago’s Salvador Eye Hospital, reported that only 20% of the projectiles were made of rubber while the remainder comprised silica, barium sulfate and lead.

Many protesters say they have been blinded by rubber bullets and tear gas canisters. Patricio Acosta, president of Red Cross Chile, said more people had lost the use of an eye in the past three weeks than in the last 20 years.

Rozas said a test by the police’s own laboratory was not consistent with the university report, but had “shown differences” with the manufacturer’s technical specifications.

The “rubber pellets” used by police are cartridges of 12 pellets each, measuring eight millimeters and weighing seven grams. Police have not confirmed the details of the manufacturer.

Chile’s unrest was sparked by a hike in metro fees but spread to encompass pent-up grievances over income inequality and soaring living costs.

The protests have left at least 23 dead, around 7,000 detained, and 2,391 demonstrators and more than 1,000 police officers injured, according to authorities and rights groups.

The Chilean Ophthalmological Society said it had registered 221 instances of severe eye trauma from Oct. 19 to Nov. 18, mostly to men with an average age of 29 and mostly due to rubber bullets. In 47% of cases, it said the result was severely compromised vision.

“We will continue to reiterate to all the competent authorities, in all the necessary instances, the urgency of reviewing the protocols around safeguarding public order and stopping the use of these weapons,” the society’s president Dr Dennis Cortés said in a statement.