Substances dumped into a river will end up in the sea. The biggest pollution problem is global warming. This happens when greenhouse gases, such as CO2 are released into the atmosphere, trapping heat and causing the planet to warm up.
Chemical pollutants have been responsible for affecting the reproductive organs of fish, alligators and polar bears, stopping them from making babies.
The greenhouse effect
What is the greenhouse effect?
About 30 gases are made by human activity have been identified as contributing to the greenhouse effect. These are sometimes called greenhouse gases. The main gases are carbon dioxide, methane, CFCs and nitrous oxide. Sunlight reaches the Earth's surface, is reflected off it, and is prevented from escaping from the atmosphere by this layer of gases, which acts like glass in a greenhouse.