A 2018 study by the British Meat Producers Association determined that legally permitted levels of nitrite have no effect on the growth of the "Clostridium botulinum" bacteria which causes botulism, in line with the UK’s Advisory Committee on the Microbiological Safety of Food opinion that nitrites are not required to prevent "C. botulinum" growth and extend shelf life. In some countries, cured-meat products are manufactured without nitrate or nitrite, and without nitrite from vegetable source. Parma ham, produced without nitrite since 1993, was reported in 2018 to have caused no cases of botulism.