Smoke from this week’s conflagration has been intense, obscuring the sun, bloodying the moon – and compromising human respiration significantly.

The Bay Area Air Quality Management District (BAAQMD) calculates air quality in the greater Bay Area, quantifying five major pollutants daily: ground level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and particle pollution.

It is the particle pollution that’s been most dangerous this week, with fires in seven counties contributing to the pollution. Toxic smoke from the rash of fires has blackened skies from Davis to Alameda, putting countless individuals at medical risk.

Particulate matter small enough to lodge in the lungs is called PM2.5, and paper dust masks offer zero protection. Those masks are designed to block large particles, like sawdust, not filter out nearly invisible PM2.5. Scarves and bandanas are useless, too.

The scale for calculating PM2.5 runs from zero to 500, though levels are sometimes measured “beyond index.” People with heart or respiratory issues are endangered by air quality quantified at 100; everyone else is at risk when quality degrades to 151. Last Monday, the air quality in Napa was calculated at 442. In Santa Rosa, where the most destructive fires raged, BAAQMD had no monitor – though few needed bureaucratic metrics to recognize that the air they were breathing was neither healthy nor safe. Irritated eyes, nose and throat; coughing – both productive and dry; tightness in the chest; and shortness of breath: all are effects of PM2.5 contamination. Those with chronic respiratory issues likely found symptoms immediately worsening, but a symptom-free status was no guarantee that harm was not done.

Concentrations of PM2.5 even accumulate indoors, but can be filtered with air cleaners. If a home is not equipped or only partially equipped with air filters, designated “clean rooms” – with few windows and doors – are recommended for sleeping.

Otherwise, common sense rules the day: stay indoors with filtered air and avoid exertions that cause deep, rapid breathing; reduce other sources of indoor pollution – such as wood fires, candles, even running the vacuum. When going outside is simply unavoidable, disposable respirators marked N-95 or P-100 are effective for pulmonary protection.

Midweek, the notoriously dirty skies over Beijing, China, rated 25 PM2.5. In the Sonoma Valley, according to the website weatherunderground.com, that same measurement yielded a rating of 403 PM2.5, a level identified as “hazardous” to health.

Email Kate at kate.williams@sonomanews.com.