Take the flood in Mississippi in 1996 as an example. We can see from aerial photos that the floodwater flowed down the rivers and out into the Gulf of Mexico. Then the currents of the Gulf Stream took that water across the North Atlantic Ocean. Six weeks later, that water was off the coast of Newfoundland. The rivers, seas and oceans are not separate, they mix together.
Air also moves from one place to another.
And these chemicals stay around for a long time. Many of these chemicals take a very long time to break down in the body. Most of Europe, the United States, Japan, China, Australia, Argentina, Chile, and South Africa are in the temperate zones. Well the direct effects of people using some of these chemicals are terrible. It is particularly bad for farm workers in the Third World who don't have protective clothing and other ways to protect themselves.