When the California wildfires start I always notice the pleasant smell of a campfire smoke first. In time, this smell becomes less pleasant as it lingers. Near a fire the smoke can make it difficult, or impossible to see. It can also spread for miles and the weaker, less dense smoke, can still be harmful to people and animals, especially the elderly and young children.
Unlike a campfire, forest fires burn far more than trees. The smoke can be filled with toxins from everything caught in the path of the fire; from rubber tires to deadly chemicals. The smallest particulate matter is called PM2.5 because it has a diameter of 2.5 micrometers or smaller. PM2.5 is particularly harmful to your health.