THE pollution of our rivers by untreated sewage has been much in the news recently.

It is indeed hard to understand how the monitoring and control of such outflows has been so deficient in recent years.

News about sewage catches the headlines, but we should remember that nature knows how to deal with natural substances in moderate quantities.

The greater cause for concern is the release into the environment of countless man-made substances.

These include industrial chemicals, pharmaceuticals and personal-care products, plastics (including microbeads and microfibres), airborne particles, engineered nanomaterials, and fertilisers and pesticides used in agriculture.

Some of the hazards are obvious. Many thousands of farm workers, across the world, are poisoned by agricultural pesticides, with estimates of more than 10,000 fatalities a year, worldwide.

Other effects of chemicals are less obvious. DDT was at one time widely used as an agricultural insecticide and to combat insect-borne diseases such as malaria.

It came to be realised, however, that it had adverse effects on wildlife, as well as potential risks to human health.

Extremely low levels of DDT in rivers and lakes become concentrated in plankton, which absorb the chemical with their food.

It becomes further concentrated up the food chain, and in the bald eagle,, America’s national bird, it interferes with the metabolism such that eggshells break and no chicks are hatched.

The population has recovered since the banning of DDT.

Another example is diclofenac. This pharmaceutical product can be beneficial for humans and for livestock.

However, its use to treat cattle in India in the 1990s led to the death of vultures that had fed off the carcasses and to a consequent proliferation of other scavengers (dogs and rats) and the consequent spread of disease.

These, and plenty more, are examples that we know about.

Henry Haslam is the author of ‘The Earth and Us’.