All wood smoke is a human health problem. There are conditions
that make it even more deadly. Many communities are located in
valleys. When surrounded by mountains, hills or trees there is
little movement of the air. Smoke is heavy and tends to sit close
to the ground especially under low wind conditions. The smoke
reduces visibility as it builds. Another contributing phenomenon
in many areas, including the SF Bay Area, is the Temperature Inversion.
It happens on clear, cold, still evenings and is caused by radiation
cooling of the ground which occurs faster than the cooling of
the air right above ground giving you a layer of warm air above
cooler air. The rather sharp boundary tends to prevent mixing
between the layers. In the Bay Area, the inversion layer rises
to only about 80-100 feet before dissipating in the early morning. It thus acts as a trap for any smoke or other air pollution that
occurs in the evening.
There is no sunlight to degrade the chemicals emitted and warm air acts like a lid over the colder air, as the air at the ground cools while the sun goes down. The cool air also slides down the valley walls, pooling on the valley floors. This prevents vertical mixing. The cold air layer rises to 80 or 100 feet above the ground. The wood smoke and any other pollution generated at this time remains within this limited amount of air. Severity of the temperature contrast and the amount of wind, with no cleansing rain, can allow stagnant conditions lasting for days, as you saw in Particulate Pollution as Measured Out-side a Los Altos Home in January 1992. Individuals cannot control how bad the pollution will get or how long it will last before being cleansed. This is the only air there is to breathe.
Rising PM 2.5 air pollution is a public medical emergency, yet the US EPA requires that cities monitor it every sixth day. They do not require that it be monitored by time of day. High levels of particulate are caused by 2% to 7% of the people in the Bay Area. The emergencies occur on the weekends and holidays when the air district offices are closed.
Picture of air inversion from CA ARB Wood Burning Handbook.
Pollution levels in SF Bay Area from residential burning usually peak by midnight inside the non-burning home. By the time it filters out hours later, many people are getting up and ready for work. Then they will be in traffic pollution and then possibly business pollution for the rest of the day. If they work in an area with restaurants cooking over solid fuel they will have wood smoke in their work place. When fires are set inside or outside the home or business the smoke attacks many victims. It degrades the quality of life for the entire community. Similar PM 2.5 pollution patterns are seen around the country and the world.