Not all green is healthy

RABINDRANATH Tagore's much loved tree, the scholar's tree or Alstonia scholaris, is today a bone of contention within the faculty of the Indian Institute of Technology (11T) in Kanpur.

Last year, when about 10 Alstonia trees were felled on the campus after several staff members complained of asthma attacks, it provoked much protest from environmentally conscious staffers. The IIT campus has nearly a thousand tall, evergreen Alstonias.

A B Singh, an allergy expert at the Centre for Biochemicals (CB) in New Delhi, however, claims that there are as yet no studies indicating that Alstonia pollen is an allergen. But he indicates a number of trees that do stimulate allergic reactions in susceptible individuals. He complains that numerous exotic trees are nonetheless being planted under afforestation programmes without evaluating them for their allergen potential.

Singh cites the example of mulberry (Morus alba) which is planted extensively in India while, because of its allergenic pollen, its plantation is prohibited in certain arid regions in USA. Not all green is good for human beings, he emphasises.

Singh's concern is understandable. Ten per cent of the Indian population today is affected by major allergic disorders. Of them, about four per cent suffer from allergic rhinitis and over one per cent from bronchial asthma.

Trees with small flowers produce abundant dry pollen. Though the trees flower at different times of the year, their pollen is usually dominant during the FebruaryApril season. The August-October season is dominated by pollen from grasses and weeds.

However, there are exceptions. For instance, Prosopis juliflora, an exotic species used extensively in Gujarat and Rajasthan for afforestation, has two peak pollination seasons in a year. Flowering periods of trees thus vary across the country, depending on local ecological and climatic conditions. The allergenic protein content of pollen from the same tree species can also vary from one region to another, depending on local humidity and temperature. P Malik and his colleagues at the CB have found that the protein concentration of Holbptelea integrifolia is more than double in Delhi than what it is in Bangalore. So it is externely important to develop locale-specific allergen pollen calendars to help asthmatics.

Most allergenic plant species are endemic to India, but some like Prosopis juliflora, Casuarina equisetifolia and eucalyptus have been introduced recently. Another example is partheniurn or Congress grass, a gregarious exotic weed which has spread rapidly to almost all parts of the country since it was introduced with the import of PL 480 wheat from USA. This weed is a known allergen and afflicts people in semiurban environments.

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