As a young scientist, I was deeply impressed by Rachel Carsons' book - 'The Silent Spring' which warned against the damage that synthetic carcinogens could do to us and our environment. But today, in the U.S. at least, synthetic chemicals account for a minute proportion of cancers in humans. Similarly, inherited genetic factors affect mainly children and young adults. In adults, most heart disease and most cancers are caused by tobacco, chronic infections and what we don't eat.

The quarter of the population that eats the least fruit and vegetables has twice the death rate from cancer of the quarter that eats most. Between 40% and 60% of the more than 500,000 cancer deaths in the U.S. last year could have been prevented by good diet, antioxidant supplementation and the avoidance of tobacco products.

With what we know today, our life span and our health may be extended by the good choices that we make as individuals. At least as far as long and productive old age is concerned, we are not prisoners of our genes. Chronic illness is not pre-destined genetically. The science of the last decade is helping us understand how our personal choices can maximize our potential to live out the maximum human life span.

The new developments in genetics are exciting and it is possible that the 120-year barrier may be breached by new understandings. Most human cells undergo a finite number of cell divisions before entering a non-dividing state called replicative senescence. The geneticists have uncovered several molecular clocks which may limit the number of times a cell may renew itself by division and thus set limits to life span. Telomeres are tags on the end of chromosomes which are shortened each time a cell divides. It has been proposed that telomere shortening is the molecular clock that triggers senescence. There are now reports emerging linking telomere mechanisms to redox-modulated processes and oxygen toxicity.

Other scientists are working on laboratory-grown organs to replace worn out parts and on rejuvenating hormones. We can expect new therapies for heart disease and cancer to follow the deciphering of the genetic blueprint from the human genome project.