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4. Toxicity, humans and the environment

Any substance is potentially toxic if the dose and duration of exposure are sufficiently high. However, there are many ways in which chemicals might disrupt the functioning of an organism, including corrosive or irritant effects, acute and chronic toxicity, effects on the nervous system (neurotoxicity), impairment of the reproduction of cells or organisms (by carcinogens, mutagens or reproductive toxins), or damage to hormone systems, for example, the effects resulting from endocrine-disrupting chemicals (RCEP, 2003). Tests have been devised to assist in determining toxic doses - i.e. doses required to cause a specified impact on health of humans or other living organisms. The major sources of uncertainty in toxicity testing include the difficulty in envisaging all important possible impacts, and in modeling or otherwise determining the conditions under which the relevant doses are likely to be applied. Regarding the former, the relatively recent realisation that endocrine disruption is important is a reminder that there may still be impacts that have yet to be recognised, and for which tests have not yet been developed. Regarding the latter, in general, tests are devised which are intended to take a conservative approach in identifying 'worst case' scenarios of doses, accompanied by risk assessment based upon comparing the estimates of concentration in different conditions.

It is well-documented that toxicity is not solely a characteristic of synthetic chemicals (e.g. RCEP, 2003). Some of the most toxic substances known occur naturally in organisms, where they usually form part of a defence mechanism. For example, many plants, including common ones such as clovers, produce hydrogen cyanide when damaged, and a number of Australian plants produce fluoroacetate, a respiratory inhibitor which is highly toxic to sheep, but to which red kangaroos have adapted.

Therefore, in devising toxicity tests, and levels of exposure above which toxicity is deemed to cause an unacceptable impact, due cognisance must be paid both the natural and synthetic substances, and to the conditions of the receiving environment. Characterisation of the dose-response relationship is thus a process of identifying the concentration and time of exposure required to produce a specific impact outcome, or response. Hence, we can talk of the dose-response relationship between airborne lead from vehicle exhausts (assuming leaded petrol) and the response in terms of child brain development impact. Of course, the dose for another response, such as death of roadside vegetation, will be different.

In WEEE-relevant substances, it is important to note that the form of the waste and the conditions of its storage or burial will be important in determining both the dose and the response. As there are a wide variety of landfill conditions, not to mention illegal tipping conditions, the determination of dose-response across these varying conditions is not simple, and even if it could be determined, it would vary across the various conditions, and for different receptors, impacts and outcomes. A common approach is therefore to set 'guideline' levels of concentrations of known toxic substances, above which further investigation or treatment is required. Where dose-response relationships are unknown, it is appropriate to take a precautionary approach and set exposure levels to as low as reasonably practicable.

To provide a general overview of the 'toxicity' of the substances under review in this study in a New Zealand context, New Zealand Landfill Leachability Limits are included in Table 2, and a summary drawn from a recent review (Table 3) below, with further explanation in the following subsections.

Table 2. New Zealand Landfill Leachability Limits

All figures are sourced from the NZ Ministry for Environment Leachability Limits for Class A and Class B Landfills.

WEEE Substance NZ Landfill Leachability Limits

Lead

Class A: 5mg/L. Class B: 0.5mg/L

Mercury

Class A: 0.2mg/L Class B: 0.02mg/L.

Cadmium

Class A: 1 mg/L Class B: 0.1mg/L.

Barium

Class A: 100 mg/L Class B: 10 mg/L.

Hexavalent chromium

Chromium VI Class A: 5 mg/L Class B: 0.5 mg/L.

Beryllium

Class A: 10 mg/L Class B: 1mg/L.

Brominated flame retardants

Not indicated.

PVC (polyvinyl chloride)

Vinyl chloride Monomer: Class A: 2 mg/L Class B: 0.2mg/L.

PBDE's (polybrominated diphenylethers)

Not indicated

Phosphorus

Not indicated

http://www.mfe.govt.nz/publications/waste/haz-waste-guide-module-2-may04/html/table-appendixa.html