A major multi-disciplinary study was undertaken by ESR for the Ministry of Health1 to investigate possible historical exposure to dioxin, a by-product of the manufacture of 2,4,5-T at the IWD plant (now Dow AgroSciences) in Paritutu, New Plymouth.

2,4,5-T was produced at the IWD plant in Paritutu from 1962 to 1987, and was used extensively throughout New Zealand for gorse control.  One particular type of dioxin, 2,3,7,8 tetrachlorodibenzo-p-dioxin (or TCDD) is a by-product of the manufacture of 2,4,5-T. 

The term dioxin herein refers to 2,3,7,8–TCDD. 

A parallel study by the Ministry for the Environment2 measured soil dioxin levels in selected sites and gave the community assurance that the area was currently safe to live in. The results indicated there was a negligible health risk from the low levels found. This meant ESR could focus on determining whether historical exposure had occurred. Social science methods ensured that representatives from the Paritutu community were closely involved in designing the best study to address historical dioxin exposures.

ESR investigated the levels of 2,3,7,8-TCDD in the blood serum of 52 residents who lived close to the agrichemical plant from 1962-1987 when 2,4,5-T was being manufactured. The blood samples were taken in two stages following a number of modelling exercises to guide selection of those most likely to have had high exposure.  These included:

• Air dispersal modelling and soil testing
• Spatial modelling of soil data
• Multi-pathway exposure modelling
• Toxico-kinetic modelling 

The final report confirmed that residents who had lived very near to the plant for at least 15 years between 1962 and 1987 were more likely to have higher levels of TCDD than other New Zealanders.  This was thought to be from inhalation of TCDD from IWD.

The 52 participants of the study had blood levels of TCDD ranging from 0.85 to 33.3 parts per trillion (ppt). The average TCDD level of this group was 6.5ppt. The expected level was 1.7ppt. (1 ppt = 0.000000000001 gram).

• Residents tested had to have lived within two kilometres east or one kilometre south of the former plant for at least one year between 1962 and 1987 and not worked at the plant or been occupationally exposed in other ways. The average age of the 52 participants (30 women, 22 men) was 58 years.
• Individual blood testing was undertaken in two rounds so that the results from the first round could assist in the selection of participants for the second round. Results were compared with those obtained in a national serum study carried out by the Ministry for the Environment (MfE) in 1996-1997.
• Across the test group, there was a mean serum TCDD concentration of 6.5 pg/g lipid (range 0.85 to 33.3) whereas the expected national mean for the same age/sex groups would be 1.7 pg/g lipid.
• Duration of residence from 1962 to 1987 was an important factor in the level of elevation. The mean TCDD level for those with at least 15 years’ residence was 14.6 pg/g lipid compared to an expected mean of 2.4 pg/g lipid.
• Eating home-grown leafy vegetables and ‘exposed’ fruits also contributed to dioxin levels found. No link was found between dioxin levels and eating home grown root vegetables, ‘protected’ fruit such as citrus, poultry/eggs or local kaimoana (seafood).
• Spatial analysis of soil data is consistent with a plume of TCDD extending about 1000 metres east and 400 metres south of the plant. The highest modelled soil concentrations are immediately east of the boundary. It is not possible to determine from the study whether these emissions were regular or episodic. 
• Results suggest the serum TCDD levels found are due to historical aerial emissions from the IWD plant throughout production years