Given the many forest fires that have been burning out of control in the western U.S. I have received many inquiries about how to deal with smoke from an aviation perspective. First and foremost, smoke can present a hazard to all pilots, but especially those flying under visual flight rules (VFR).
Smoke lowers visibility, not only at the surface, but aloft as well. It is not usual for smoke to lower flight and surface visibility to below 1 statute mile which makes flying VFR dangerous. Even under instrument flight rules (IFR), visibility may be in the low IFR flight category and below published minimums for some airports. Even more concerning is flying at night and in mountainous terrain.
So what's a pilot to do? Of course, if you don't have to fly, that's likely the best option. Second, if you do decide to make the flight it's best to be on an instrument flight plan (VFR not recommended). Also, if you have oxygen on board, consider using it even below 10,000 feet. In fact, wearing an oxygen mask is always a good approach. All smoke contains some levels of carbon monoxide, carbon dioxide and other particulate matter. Hemoglobin bonds with carbon monoxide 200 times more readily than it bonds with oxygen and often produces hypemic hypoxia. Depending on what is actually burning at the surface, smoke can contain a variety of different chemicals, including aldehydes, acid gases, sulfur dioxide, nitrogen oxides, polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, styrene, metals and dioxins. None of these are good to breathe especially if you have health issues (also consider your passenger's health).
The smoke starts off as eddies in the boundary layer. But then some of that air gets mixed further up above the boundary layer into the free atmosphere, and then it encounters stronger horizontal winds. Smoke can travel thousands of miles. In fact, some of the smoke from the western fires has even reached the east coast at this point, albeit, in low concentrations of particles.
In the early morning hours the atmosphere around the regions where the fires are burning is often fairly stable near the surface. That will trap some of the smoke keeping it close to the surface. The fires burn so hot that they often produce their own convective updrafts along with "clouds" called pyrocumulus, pyrocumulus congestus flammagenitus and