Since the crop was introduced to Britain some 25 years ago, it has expanded to cover eight per cent of the country's arable land, an area of 400,000 hectares. Over the same period, the incidence of hayfever, asthma and other allergic conditions has also massively increased; episodes of asthma reported by GP practices have increased more than five-fold since 1976. No one is suggesting that oil seed rape (OSR) alone is to blame. After all, air pollution from traffic has also increased over the same period, bringing hydrocarbons and oxides of nitrogen to our cities and low-level ozone to the countryside. But an increasing number of country-dwellers do associate their allergic symptoms with oil seed rape, particularly during its peak flowering period in mid-May.
Recent research from Dundee backs up their views. Bill McFarlane-Smith and others at the Scottish Crop Research Institute, together with David Parratt of the Ninewells Hospital and Medical School, have monitored allergic reactions among 22 residents of Stormontfield in Perthshire, Scotland. They compared 1992, when the village was surrounded by 180 hectares of OSR, with 1993, when the land was planted with wheat and barley and the nearest oil seed rape was 2.5km away.
Allergic responses - sneezing, blocked and runny noses, wheezing, coughing, headaches, tightness in the chest, and eye irritation - were twice as severe on average when the oil seed rape was present, with a discernible peak in symptoms reported by 12 of the 22 subjects between 5 May and 28 May, precisely the peak flowering time recorded for the oil seed rape crop around the village.
The study, conclude the authors, "has clearly demonstrated that OSR induces symptoms in a significant proportion of otherwise healthy individuals." But the precise cause of the problem has yet to be determined. Pollen may be the obvious culprit, but the case is far from proven.
A study by the Rothamsted Experimental Station, based on 1,408 admissions to the Deddington Health Centre in Oxfordshire, found that just 20 people (1.4 per cent) had allergic symptoms that they attributed to oil seed rape, and in only three of them could the allergy be traced to oil seed rape pollen (0.2 per cent of the sample). "It is clearly not a potent allergen," says Dr Margaret Blight, co-author of the report.
But the study also examined 37 Rothamsted researchers who regularly handled oil seed rape, and found that 29 of them (78 per cent) were coughing or wheezing, or had reddened, itching eyes. Yet only five of those (17 per cent) showed specific allergic responses to oil seed rape pollen in formal tests - Dr Blight among them, having been sensitised to the pollen after several years of unusually heavy exposure.
So are McFarlane-Smith's results at fault, or could there be another allergic factor at play? Dr Blight surmises that "psychological effects" could be responsible. "Oil seed rape is so apparent due to its bright yellow colour and heavy odour," she argues, "so people naturally tend to associate their symptoms with it - but in reality, it could just as well be birch pollen."
However, she concedes this might not be the whole story. "The flowers produce a lot of chemical odours, and many of these are known irritants. They may prime the nasal mucosa to make people more sensitive."
This is the line of research pursued by Bill MacFarlane-Smith, both in the laboratory and most recently in the field. "Some people clearly are affected by pollen and fungal spores off the crop, but this does not account for the numbers who are suffering," he says. "Oil seed rape puts out a huge range of volatile compounds - such as aldehydes, terpenes, thiocyanates, isothiocyanates and glucosinolates - that have a long track record in industry as irritants."
It so happens that these chemicals, especially the smelliest sulphur- containing ones, are put out in peak volumes at the same time as pollen releases are at their maximum. It is not just the chemicals emitted that are known irritants, but their oxidation products such as acetone and formaldehyde. "We know that these compounds are irritant at high concentrations but no one has yet done the research to find out what they can do at lower levels," says MacFarlane-Smith. "We also need to find out if these volatiles are sensitising people to other factors, such as pollen, to produce allergic reactions."
If particular chemicals are identified as responsible, he believes the problem could be solved. "Using genetic engineering, we should be able to alter the crop to produce less of the irritants." This could bring a secondary benefit: odorous compounds attract pests, forcing farmers to spray the crop. "With fewer volatiles," says MacFarlane-Smith, "you should be able to cut down on insecticides into the bargain." !Reuse content