One of the world's largest dioxin exposure studies has found that workers at the former 2,4,5-T manufacturing plant in New Plymouth have blood dioxin levels well below the accepted level of health concern.

The University of Otago study of employees at the Dow AgroSciences (formerly Ivon Watkins-Dow) plant showed that people who worked there between 1969 and 1988 have the same life expectancy as the general population of New Zealand. The study also showed no dioxin-related increase in cancer or other diseases.

Department of Preventive and Social Medicine researcher Dr David McBride says blood samples were taken from 346 former and current workers now living in Taranaki who agreed to participate.

"As expected, many workers directly involved with 2,4,5-T manufacturing had increased dioxin levels, ranging from 1-100 ppt (parts per trillion) with an average of 10 ppt," Dr McBride says. "But even these levels are well below comparable 2,4,5-T workers in a recent overseas study, where levels ranged up to 176 ppt."

The levels are well below the level of concern recently published by the US Centers for Disease Control (CDC), which reported that increased incidence of cancer in occupationally exposed workers may be associated with dioxin levels of 495 to 31,800 ppt.

Dr McBride says that levels found among people in unexposed jobs, such as those who worked in offices on the site, were generally low by New Zealand and international standards.

"Among the 105 volunteers whose work history indicated no potential for job exposure, the dioxin level found ranged up to 31 ppt and averaged 5 ppt. There were a few people in this group who reported potential dioxin exposure outside the New Plymouth site, such as spraying on a hobby farm.

"Apart from three such individuals, the average in the unexposed group is 4 ppt. This is very consistent with background dioxin levels for the comparable age groups in the New Zealand population," Dr McBride says.

Professor Emeritus at the University of Auckland School of Medicine Sir John Scott – representing the independent expert science panel which advised the study team – says the results were positive, especially for the residents of New Plymouth.

"The scale of this study and the very high participation rate by local people mean that we now have a definitive assessment of dioxin exposure in New Plymouth. The fact that the unexposed group have dioxin levels in line with the rest of New Zealand should be very assuring for the people concerned and the general population of the city."

Dow Chemical Company Chief Epidemiologist Dr Jim Collins was a member of the study team. He says the research showed exposure levels among the local workers were low compared with studies done elsewhere in the world.

Dr Collins says Dow intends updating the study on a regular basis to ensure ongoing monitoring of the health of workers.