Acrolein, carbon monoxide, formaldehyde, and benzene are all constituents of smoke which have the potential to be harmful to humans when present in sufficient quantities. The danger these substances pose depends on the duration of exposure, and the quantities present (i.e. their concentration). Symptoms of exposure to these chemicals can range from tearing and coughing in cases of mild exposure, to death in cases of extreme exposure, such as in the case of carbon monoxide exposure. For carbon monoxide, the effects are well documented, and it becomes a concern the closer you are to the source (in this case, the flaming front of the fire). Typical signs and symptoms of CO exposure at various levels can be seen in the figure below:
For the case of acrolein, benzene, and formaldehyde, research is still ongoing as to the hazards these may pose in the concentrations present in wildland fire smoke. Click on the image below for a list of the various symptoms of exposure to these substances.