Crops grown in glasshouses and poly tunnels contain higher levels and numbers of pesticides than those grown in open fields.

Researchers from Lancaster University analysed pesticide data in food taken from various points in the food chain. Produce was “paired up”: e.g. strawberries, typically grown undercover, were paired with berries such as gooseberries, typically grown outside; the number and level of pesticides present in each sample were measured and the results compared.

Key findings include:
* Crops grown under protection have a significantly greater number of detectable pesticide residues than similar crops grown in the open.
* Average concentrations of the pesticides that typically degrade in sunlight (photolabile) were
higher in the crops grown under cover compared to field crops.
* Greater exceedances of maximum residue levels were evident in lettuce grown under cover than in field-grown cabbage.

“Consumers are very interested in pesticide levels in their food, so growers also take a very keen interest in keeping levels to a minimum – even when this is well below acceptable levels,” comments Lancaster University's Dr Crispin J. Halsall, lead author of the study, which has been published in the journal Chemosphere. “At the same time, we are developing techniques that are increasingly sensitive and good at detecting small changes in pesticide levels. This new research will help build up a clearer picture of how pesticides behave under glass and in poly-tunnels.

“Pesticide-use regimes in closed-cropping systems should be reviewed to help reduce post-harvest residues on salad and soft fruit.”

Diana Barnett is the director of SevenCanaries (, a marketplace and information resource for allergy-free and non-toxic living.