Environmental campaign group Extinction Rebellion says it is taking “full responsibility” after demonstrators spread ampoules of butyric acid in a branch of the H&M clothes store in Naaldwijk on Friday afternoon, leading several people to be treated by ambulance crews.

“We are extremely shocked, and this was not our aim,” XR said on social media. “We also used a harmless substance. We will discuss what happened among ourselves and how we can ensure it does not happen again.”

A journalist for the AD newspaper was in the shop when the chemical was released, but it is not clear from the reporting exactly what happened. Police told the paper that five to ten people – both shop staff and shoppers – had “become unwell” and were checked over by ambulance crews.

Two people have been arrested and charged with vandalism, the AD reported.

Butyric acid is created by rotting butter and smells strongly of vomit. It has been used by demonstrators in the past to create stink bombs, most notably by Sea Shepherd against Japanese whaling crews.

According to health and safety experts, the vapour can irritate the eyes, nose, and throat, and if inhaled in high concentrations, it can cause coughing and difficulty breathing.

The protest is part of a campaign against fast fashion shops, which has already been ongoing in France and Belgium. The campaign Ode Aux Rats is derived from claims that fast fashion stores are “experimenting” on shoppers by blasting different fragrances through ventilation systems to encourage them to buy more.