The San Francisco Bay Area’s skies were the worst on record for decades during the recent wildfires that produced a blanket of smoke and turned the skies orange, according to pollution measurement firm Aclima.

Aclima has placed sensors on a fleet of electric cars to collect daily pollution levels on a block-by-block basis in the Bay Area. For the past month, six of the largest 20 fires in California history poured smoke into the atmosphere, and new fires in Oregon and Washington also contributed to pollution levels that were the worst on record. This analysis is based on regulatory data showing a record number of consecutive Spare the Air days which go back to 2001.

Ash coated cars in Silicon Valley, and entire towns in the fire zone had to be evacuated. More than 3.5 million acres have burned from 7,900 wildfires. More than 5,800 buildings have burned, and 26 people have died.

Aclima said the fine particulate matter (PM2.5) levels on the ground had dropped some from late August to September 9. But PM2.5 concentrations quickly increased the second week of September as the blanket of smoke above mixed into the air we breathe on the ground.

The company’s mobile sensing network measures and analyzes air pollution and greenhouse gases block-by-block (hyperlocal), weekdays and weekends, day and night, covering more than 5,000 square miles throughout the Bay Area. The analysis you see in the video draws from an aggregation of billions of preliminary data points Aclima generated. The PM2.5 concentrations over time were quite variable from August 16 onward.

The skies above San Francisco turned orange on September 9 (delineated by the orange bar in the graphs). Between counties, there tended to be more variation in maximum daily levels than in daily averages. Often, the readings of real-time sensors operated by the Bay Area Air Quality Management District, Air Now, and Purple Air didn’t match what people saw in the skies. On the day that the sky was orange, for example, the air was relatively clean.

That’s because, in the Bay Area, an inversion layer of cool marine air blew in from the ocean and was trapped beneath a layer of warmer air. The marine layer was stable, meaning there is no exchange of air between this lower level and the air above it. That trapped the bad air high in the sky, according to Aclima chief scientist Melissa Lunden, in an interview with VentureBeat.

The sky was orange that day because the smoke particles scatter sunlight differently depending on the wavelength of the light. Normally, the gas molecules in the air scatter the blue light in all directions, and this scattered blue light is what we see. The smoke particles completely filter out the blue light while the longer-wavelength orange and red light gets through to our eyes.

While the orange glow was gone quickly, the smoke remained. Similar daily averages and maximums were found across all Bay Area counties, suggesting that the smoke was evenly dispersed across the entire Bay Area during this time period. The downward trend from September 14 to 15 continues through today.

In most California counties, both daily averages and maximums stayed relatively low compared to previous weeks until September 6. After that most levels rose, coinciding with several rapidly spreading fires including the Creek Fire and the Bear Fire, as well as fires outside of California.

Average daily ozone levels actually decreased as the smoke blanketed much of California. As PM2.5 levels rose, ozone levels began to drop on September 9 throughout the state. Ozone is typically elevated in summer due to photochemical reactions in the atmosphere triggered by sunlight and heat that generate ozone from other gases. In this case, the blocking of the sun and the subsequent cooling of the ground and air beneath the massive blanket of smoke likely contributed to declining ozone levels over the past week in many parts of the state.

Despite these decreases in ozone concentrations, sustained unhealthy levels of PM2.5 have prompted alerts from air quality agencies to stay indoors and avoid outside activities. The Bay Area experienced its longest sustained period of unhealthy air on record, emerging on Thursday from a 30-day streak of consecutive Spare the Air days.

The lightning storms, fast-moving wildfires, orange skies, falling ash, unhealthy air, and even declining ozone levels all serve as a stark reminder of the compounding impacts and unintended consequences of environmental destabilization, Aclima said.