Concrete was found in drugs tested on-site at Cumbria’s Kendal Calling Festival at the weekend.
The potentially fatal ingredient was among numerous foreign chemicals, including malaria medicine and insecticide, found by charity The Loop, which offers drug testing without users facing arrest or criminal charges.
One festival attendee who used the service told the Independent: “I read some bad reports about the pills as well as some good ones, so I decided it would be best for me that I have the reassurance in my mind to know that I've come here, have it tested and know for sure.
“It just gives me peace of mind to know that what I'm taking is safe instead of just taking anything."
The Loop, a not-for-profit organisation, had a number of festivals sign up to use its services this summer including Reading and Leeds.
The Loop director, Professor Fiona Measham, said: "We accept that some people will get drugs on site and some people will be planning to take them so what we're doing is trying to address any potential health problems.
"This is a focus on public health rather than on criminal justice."
She aded that about one in five users bin their gear after testing.
Read our interview with Fiona Measham here.
Louis Anderson-Rich is Mixmag's Digital Intern. Follow him on Twitter