The T-value in PBT refers to acute toxicity.
There is a risk of selecting resistant bacteria with the concentrations measured in Swedish wastewater treatment plants. However, the estimate of selective concentration is very uncertain for metronidazole and measurements have not been taken in recent years. Metronidazole has previously been found in purified wastewater and surface water in Stockholm County (2005–2012).
The risk classification insignificant, according to Fass.se, is based on total sold amount (kg) of the substance in Sweden during the year 2014 and the toxicity of the substance.
Comparative assessment of environmental risk and risk of resistance selection in human use of ivermectin, metronidazole, lymecycline and azelaic acid in Sweden (Report Goodpoint 2018). The risk is clearly highest for lymecycline based on measured concentrations of tetracycline in uncleaned waste water that exceeds experimental established selective levels for antibiotic resistance in bacteria. A similar but lower risk profile exists for metronidazole based on total use but with significantly lower empirical support for the efficacy The topical use of metronidazole results in a negligible risk of resistance selection in waste water treatment plants. The risk of effects of human use of ivermectin is also low, given an expected contribution to exposure in aquatic environments at subnanogram levels, and a majority of efficacy studies that show significantly higher levels of efficacy. Since there is (at least) a study that reports effects at 1 pg/L, however, one can not completely rule out risk. For azelaic acid there is nothing that indicates an environmental hazard.
Author: Health and Medical Care Administration, Region Stockholm