Fossil fuels are burned to release energy, but when we burn these fuels it releases harmful pollutants into the atmosphere that can harm the environment by harming plants, animals and causing acid rain, and it can be dangerous for us as it can harm our lungs.
All fossil fuels contain a lot of carbon so carbon dioxide, carbon monoxide and carbon particulates are formed (these are all pollutants). If there's enough oxygen while the combustion is taking place then all the carbon should end up in the atmosphere as carbon dioxide. If there's not enough oxygen then there is incomplete combustion and carbon monoxide and carbon particulates are formed.
Carbon dioxide will stay in the atmosphere causing us problems until it can be removed. It can be removed naturally by photosynthesis and carbon dioxide also dissolves into rain water, rivers, lakes, oceans and so on. If the carbon dioxide level increases then the greenhouse effect will increase as well. The carbon dioxide is not allowing extra heat to escape causing the Earth to warm up. This is causing problems like the sea levels rising.
Carbon monoxide is very poisonous as it decreases the amount of oxygen your blood can carry so less oxygen is getting to your brain…