People have used chemicals against living things that make their lives harder for hundreds of years. By and large, until recent times, they were rather ineffective. Before modern transport, there was no point in a farm growing the same crop (e.g. beans) in every field, as he could only sell to the locals and there were only so many beans they could eat. So, every farm grew a bit of all sorts and rotated their crops around the fields. The pests struggled to find the crops they ate and their number rarely grew to problematic size. As transport improved it allowed farmers to concentrate on one crop, the same in every field. Pests didn’t need to move to search them out and the number of pests rose and rose. To combat the rising number of pests, pesticides were made increasingly more deadly.

Since the mid-20th century, we have discovered extremely poisonous chemicals (mostly made from oil) that kill on contact (they don’t have to be eaten). This is far more effective in killing most things; and they are sold as pesticides. Each new pesticide seems to be more potent than the previous and new ones are created as one by one the ones in previous use get withdrawn; as governments are made aware of the immense damage they do.

Probably the most famous example of a once widely used, but now largely banned, pesticide is DDT. At one time humans’ breast milk contained so much DDT that had the same quantities been in cow’s milk it would have been declared unsuitable for human use. Population means of DDT in breast milk declined throughout much of the world, from 5000-10,000 mcg DDT/kg milk fat to around 1000 today in many areas. The decline observed in some countries nonetheless corresponds to their restriction of DDT use. Levels can still be high in areas still using DDT, even higher than the World Health Organization’s recommended ceiling for infants. Read the full article on Worldwide trends in DDT levels in human breast milk.