Perchlorate levels doubled in organic veg
21 November 2005
Organically grown leafy vegetables produced in North America contain double the amount of perchlorate found in conventionally grown crops, report US researchers.
Perchlorate, a by product of rocket fuel production, is present in groundwater and surface water throughout the US. It presents a health risk to humans because it limits iodine uptake by the thyroid gland. The main exposure routes are drinking water and crops that have been exposed to contaminated water.
The researchers, following up on recent reports of perchlorate levels in lettuces irrigated with Colorado River water, measured nitrate and perchlorate in various types of lettuce and other leafy vegetables from across North America. Charles Sanchez from the University of Arizona and colleagues studied a selection of organically grown and conventionally grown produce.
The researchers were surprised to find that twice as many organically grown as conventionally grown samples contained measurable levels of perchlorate. Also, the average concentrations in the positive samples were twice as large for organic crops. The team has no real explanation for this, but has discounted fertilisation as a possible mechanism.
Health risks from these elevated levels of perchlorate appear negligible. The estimated daily exposure for adults eating organic leafy vegetables is 2 mg/day, which is less than 10 per cent of the recommended daily intake.
The perchlorate data were compared with data on nitrate, another iodine uptake inhibitor. Perchlorate is 240 times more potent than nitrate as an uptake inhibitor, but the relatively large nitrate content in crops ensured that the iodine inhibition potential of total perchlorate was about 100-fold lower than that of total nitrate.
'The research will be extended to estimate potential hypothetical exposure from a number of food crops irrigated with Colorado River water,' Sanchez told Chemistry World. 'It will include broccoli, cabbage, cauliflower, carrot, onion, citrus, cantaloupes, watermelon, tomatoes, peppers, and squash.' Steve Down
C A Sanchez et al, Environ. Sci. Technol. 2005 (DOI: 10.1021/es050804k)