Pesticides such as DDT and hexachlorocyclohexane, as well as industrial waste, mainly from the Baikalisk pulp and paper plant, are thought to have exacerbated several disease epidemics among Baikal seal populations. The chemicals are speculated to concentrate up the food chain and weaken the Baikal seal's immune system, making them susceptible to diseases such as canine distemper and the plague, which was the cause of a serious Baikal seal epidemic that resulted in the deaths of 5,000–6,500 animals in 1987–1988.[1][2] Small numbers died as recently as 2000, but the reason for their deaths is unclear.[3] Canine distemper is still present in the Baikal seal population, but has not caused mass deaths since the earlier outbreaks.[1] In general, levels of DDT and non-ortho PCB have declined in the lake from the 1990s, levels of mono-ortho PCB has showed no change, and perfluorochemical increased.[1] Industrialization of the area near Lake Baikal is increasing and future monitoring is necessary. At present, Baikal seals show lower levels of contaminants than seals of Europe and North America, but higher than those in the Arctic.[1]