Exposure time, 20-year aging tests

Mention should also be made of the 30 year ageing data on PVC, UPVC, HDPE, LDPE and a styrene-butadiene copolymer tested by Rapra in hot-dry and hot-wet environments shielded from sunlight, published by Brady and co-workers [25]. Exposure for such long periods is very rare. Dioctyl phthalate plasticised PVC, HDPE and LDPE showed little change and of the remainder loss of strain to failure was more noticeable than loss of strength. Updated results from this 40 year programme are not in the public domain at the time of writing. 

Multigenerational Study. This approach involves the continuous exposure of a rodent species (usually mice). The parental animals are exposed shortly after weaning (30-40 days of age). At reproductive maturity, the animals are mated. The first generation is produced (Ff. From these an F2 is produced and then subsequently an /) generation. The effects of the test is monitored through each generation. The measured parameters include fertility, litter size, and neonatal viability. This is a time-consuming effort that usually takes about two years to complete. 

Life history models may be useful for considering age- or stage-specific variability in sensitivity in the extrapolation of response however, it should not be assumed that smaller or younger stages are always the most sensitive. The few studies that have addressed the impact of time of year on responses of aquatic communities to a stressor indicate that, in freshwater communities, threshold concentrations for direct toxic effects vary within a factor of 2 among seasons — well within the normal range of variation observed in laboratory toxicity tests. However, at greater exposures, the intensity and duration of direct and indirect responses may vary considerably between different periods of the year because of the influence of climatic and seasonal factors on recovery. 

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