Hydrogen sulphide gas (H₂S) has a distinctive sulphurous odour, reminiscent of rotten eggs. It can be formed as a by-product from kraft pulp mills, oil refineries, livestock feedlots and fish processing plants. Until April 2002 NB ENV monitored H₂S at the Hillcrest station in west Saint John. Since then Total Reduced Sulpher (TRS) has been measured at this site.