Health risk assessment and health risk management with special reference to sodium monofluoroacetate (1080) for Possum control in New Zealand

Abstract

The principal use of sodium monofluoroacetate (1080) in New Zealand is to control brushtail possums (Trichosurus vulpecula). Aerial application of baits containing 1080 is the most common method used for large-scale control of possums.

The use of 1080 attracts a great deal of controversy, in particular the effects on the environmental, non-target species, and the potential chronic effects in humans associated with environmental exposures. Although the nature of the acute toxicity of 1080 has been known for more than fifty years, little is known of its effects on humans, in particular its chronic effects to environmental exposures.

A benchmark dose (BMD) as an alternative to a no-observed-adverse-effect-level (NOAEL) approach was investigated as a means to improve current health risk assessment values of 1080. Both approaches were investigated for three critical toxicological end points, namely cardiomyopathy, testicular toxicity and teratogenic effects identified from the few available critical studies. The calculated BMDs and lower-bound confidence limits (BMDLs) for the three end points were estimated using the Weibull, probit and quanntal linear models. A benchmark response (BMR) of 10% (extra risk) was chosen and the Akaike's information criterion (AIC) was used in selecting the appropriate model. The BMDL estimates derived were generally slightly higher but comparable to the corresponding NOAEL for those same endpoints. The computed BMD10 and BMDL10 for cardiomyopathy and testicular effects were 0.21 mg kg^(-1) bw^(-1) and 0.10 mg kg^(-1) bw^(-1) respectively. Tolerable Daily Intakes (TDIs) were derived using the NOAEL approach and the BMD methodology and applying an uncertainty factor of 3000. The resulting TDI using the BMDL were generally consistently slightly higher than those derived using the NOAEL approach. Based on the best fit of modelled dose-response data, a TDI of 0.03 μg kg^(-1) bw^(-1) day^(-1) is proposed for human health risk assessment.

Two sets of Provisional Maximum Acceptable Values (PMA V) were derived using the highest concentration of 4.0 μg L^(-1)1080 found in water (N=1450), and using the maximum allowable concentration of2.0 μg L^(-1) of 1080 in water for adults (0.58 μg L^(-1) and 0.94 μg L^(-1) , respectively) and children (0.23 μg L^(-1) and 0.4 μg L^(-1), respectively). Parameters used in the derivation of PMA Vs were average weight, average quantity of water consumed, and proportion of total intake allocated to drinking water. The derived adult PMA V of 0.60 μg L^(-1)is proposed in revising the PMA V for 1080 in the Drinking Water Standards New Zealand. This value is 6-fold lower than the current PMA V of 3.5 μg L^(-1). Additional toxicology studies are recommended to meet the definition of a "complete database" and therefore estimating a more defensible TDI, and consequently a PMAV for 1080.

Risk management approaches are consistent with the Ministry of Health's current precautionary approach. A PMA V of 0.60 μg L^(-1) in drinking water is recommended to consider it suitable for human consumption and that continuous monitoring be carried if the level of 1080 exceeds 50% of the proposed PMAV as a requirement for Priority 2 determinands in the Drinking Water Standards. Precautionary approach appears to be warranted and this was supported by information provided by the Public Health Units (PHU) where 1080 was permitted to be dropped onto drinking water catchments. The PHU s exercised precautionary measures by imposing appropriate conditions to suit local circumstances. As 1080 may likely remain an essential tool to contain tuberculosis spread by possums and to reduce possum damage to forests and crops until better methods of control are developed, a number of recommendations were proposed to protect public health.