The solutions of isohumulones can then be titrated against a metal salt such as nickel acetate or lead acetate.

Teflon, nickel acetate, cobalt acetate, and hot sodium or potassium dichromate seals are commonly used.

Hydrated nickel acetate induced lung tumours in mice following repeated intraperitoneal injection (Poirier et al., 1984; Stoner et al., 1976).

There was no increase in the incidence of tumours in hamsters exposed for up to 20 months to fly ash enriched with nickel acetate, although histopathological examination was limited to only a few organs (Wehner et al., 1981).

Similarly, there was a decrease in the number of alveolar macrophages and in the humoral response in rats inhaling 0.025 mg of Ni/m3 as nickel oxide (produced from decomposition of nickel acetate) for 4 months (Spiegelberg et al, 1984).

When this titration is done using nickel acetate, two moles of isohumulones combines with one mole of nickel acetate, and each milliliter of titrant contains 1/500 of a millimole of nickel.

The Lowest-Observed-Effect-Level (LOEL) in the identified studies was 0.1 mg of Ni/m3; at this concentration, rats exposed to nickel oxide produced by decomposition of nickel acetate had increased numbers of macrophages with more than one nucleus (Spiegelberg et al., 1984).

An increased incidence of lung tumours was observed in mice administered hydrated nickel acetate by intraperitoneal injection (Poirier et al., 1984; Stoner et al., 1976); "soluble" nickel compounds have also induced tumours at the site of injection in animals (IARC, 1990).