 A pollutant is any substance that has harmful or poisonous effects. We usually think of pollutants as being chemicals, like DDT, that are both harmful and poisonous. However, a substance that isn't a poison can still be very harmful. A common example is phosphorus, which is an essential plant fertilizer. A person would have to eat about a kilogram of phosphate to be poisoned, so it's not considered a poison. However, excess phosphate is very harmful in rivers and lakes, so it's certainly considered a pollutant. Plastic isn't toxic, otherwise we would eat with plastic forks, but it's still harmful in the environment. That's why we have rules against littering and dumping plastic trash into the ocean. CO2 is a naturally occurring gas. It is not a poison, however we have overwhelming evidence that CO2 is harmful. It is heating the earth, acidifying the oceans, and raising sea levels. These effects will displace millions of people, disrupt global ecosystems, and threaten our food supply. And unlike arsenic in your drinking water, the harm caused by CO2 extends far beyond your neighborhood and covers the whole planet. One way that some people detract from the real problem of climate change caused by carbon dioxide is to claim CO2 is not a pollutant. This is an example of the red herring fallacy. In 2007, the US Supreme Court agreed that carbon dioxide fits the definition of a pollutant, and the US Environmental Protection Agency decided that CO2 should be regulated as a pollutant because its climate effects pose a clear danger to public health and welfare. In fact, carbon dioxide stays in the atmosphere for about a thousand years, continuing to cause dangerous climate change for centuries. Most of the other substances that we consider to be pollutants don't last nearly that long. Quibbling about the technical definition of the word pollutant demonstrates the red herring fallacy. It is an irrelevant point that detracts from the real discussion. A red herring is a smelly smoked fish that was used to distract tracking dogs from the scent they were following. That's probably a myth about the origin of that phrase, but that doesn't change its usefulness. Wouldn't it be great if I could divert this whole discussion to a debate about why this fallacy is called a red herring? We could go on and on about the redness of herrings, the noses of dogs, and the smelliness of fish. Then we'd stop talking about the fact that carbon dioxide is changing our climate. The fact that carbon dioxide is changing our climate should be the focus of the discussion. Not how we name harmful substances.