Sir - In your article on biofuels (April 11), in which used cooking oil is made into biodiesel, I was horrified to read that one college chef said that they "do an oil change twice a term".

These vegetable cooking oils are known to be carcinogenic: cancers specifically linked to eating such oils include breast and skin cancers. It is also known that heating these oils increases free radical activity and with it the cancer risk. Using cooking oils just once is a risky business for the consumer, but using these oils over and over again for weeks on end is potentially hugely dangerous to health.

Now, I recognise that chefs don't usually read medical journals and I don't blame the chef quoted for reusing the cooking oil; however, surely such a prestigious university as Oxford employs academics who are qualified in this field?

Barry Groves, Milton-under-Wychwood