In 1988, a significant percentage of seals in the Baltic Sea died from viral diseases; off the coast of Scotland, however, the death rate due to viral diseases was approximately half what it was for the Baltic seals. The Baltic seals had significantly higher levels of pollutants in their blood than did the Scottish seals. Since pollutants are known to impair marine mammals' ability to fight off viral infection, it is likely that the higher death rate among the Baltic seals was due to the higher levels of pollutants in their blood.
Among marine mammals other than seals, the death rate due to viral diseases in 1988 was higher in the Baltic Sea than it was off the Scottish coast provides the most additional support for the argument.