For decades, Southern California has waged a slow but successful war on smog. Through vehicle emissions rules, clean-fuel standards and other tough measures, officials have lifted the choking pall of air pollution that once shrouded Los Angeles, bringing clearer skies and healthier lungs.
But now, progress appears to be faltering. Smog has gotten worse for the second straight year, even though emissions are on the decline.
That apparent disconnect is forcing regulators to explain why air quality in the nation's worst-polluted region is dipping.
Ozone, the harmful gas in smog that inflames the lungs and triggers asthma attacks and other health problems, has violated federal health standards 145 days this year across Los Angeles, Orange, Riverside and San Bernardino counties, air monitoring data show. That's up from 132 ozone violation days last year and 113 in 2015.
The South Coast Air Quality Management District believes the uptick is probably the result of more days with hotter temperatures and inversion layers, weather patterns that trap pollution near the ground.
Still, air quality officials are looking into other possibilities, including whether its emissions models showing a steady decline in smog-forming pollution from cars and trucks and other sources are somehow off.
"We're not denying that we've had two really bad ozone years," said Philip Fine, deputy executive officer for the district. But he added: "We are investigating to confirm our assertions that emissions are going down."
The last time the region had more than 140 bad air days was in 2004, when there was more than twice as much smog-forming pollution spewing out of tailpipes and smokestacks, according to official inventories.
Wary of officials' history of blaming pollution on changes in weather, environmentalists and public health groups say the Los Angeles region's dwindling progress on its signature environmental problem shows regulations are not tough enough. They say officials may be underestimating emissions and should be cracking down harder on oil refineries, ports and other big polluters to swiftly curb smog levels.
"My 6-year-old niece missed an entire week of school because her asthma flared up really bad this year," said Allen Hernandez, an organizing manager for the Sierra Club who lives in Fontana, where ozone pollution was unhealthy to breathe on 49 days this year. "It's not getting better. We're going backwards — we're getting sicker and this dirty air just isn't letting up."
Amid strings of hot, smoggy days over the last two summers, hospitals and asthma clinics reported more patients, especially children and seniors, seeking medical care for asthma and other chronic illnesses.
"I'm seeing more wheezing attacks in my asthmatics on the high-pollution days and I see a lot of them in the urgent care," said Dr. Afif El-Hasan, a pediatrician for Kaiser Permanente in Orange County and a spokesman for the American Lung Assn. "It's really rough on these families because the kid misses school, the adult misses work and medications can be very expensive."
In addition to the increase in unhealthy air days, Southern California has also seen its peak levels of ozone remain flat or even increase in recent years. And the smog season appears to be getting longer. An air district report shows ozone violations have begun occurring earlier in the year, with six violations in February 2016.
Bad air days have also jumped over the last year or two in other parts of California, including the San Joaquin Valley and the San Diego area, according to state Air Resources Board monitoring data.
Citing unseasonably warm weather, Bay Area regulators issued an unprecedented "spare the air" alert for unhealthy ozone pollution in late October — the latest in the calendar year they have called such a smog alert for that region.
Regulators and scientists say it is too soon to say whether climate change is driving recent increases in California smog. Researchers predict global warming will complicate efforts to clean air pollution by increasing the number of hot days, which speed up the photochemical reactions that generate ozone. But a few years of data is not enough to detect the fingerprints of global warming, they say.
Tough regulations have brought dramatic reductions in air pollution and improved health since the days when smog hung over Los Angeles in a mountain-shrouding pall in the 1970s and '80s. Ozone levels began dropping rapidly in the 1990s, when there were more than 200 bad air days a year.
But in recent years, those gains have slowed. Southern California reigns as the nation's smoggiest region and fails to meet a series of federal ozone standards going back to 1979.
In California, air pollution is regulated by state and local agencies and the U.S. Environmental Protection Agency. But state officials are resisting the Trump administration's efforts to roll back air quality and climate change rules. That included EPA Administrator Scott Pruitt's attempt earlier this year to delay the Obama administration's 2015 health standard for ozone pollution, which once implemented would require tighter regulations in areas where monitoring shows the air is unsafe to breathe.
Environmental and health groups threatened to sue after the EPA missed an Oct. 1 deadline to identify which parts of the country are in violation of the 70 parts per billion limit on ozone. The agency last week identified areas that already meet those standards but said it was "not yet prepared to issue designations" for hundreds of counties with unhealthy levels of smog, including most of California and urban areas across the nation.
Some of the nation's worst ozone pollution could be found in San Bernardino, where levels spiked as high as 158 parts per billion over the summer. Air district officials say the highest concentrations are beginning to shift from the Crestline station in the San Bernardino Mountains to more-populated areas of the Inland Empire and San Gabriel Valley, such as Redlands and Glendora, which also recorded some of the region's highest ozone readings this year.
Air quality officials say their projections show emissions of smog-forming pollutants continue dropping dramatically. And as mandates for cleaner cars and trucks continue to kick in, they expect ozone levels to decline over the long term.
Unsteady progress is expected, they say, given the difficulties in controlling photochemical reactions that form ozone and the big influence of year-to-year variations in the weather. A lack of air-cleansing storms during the five-year drought from which California emerged this year also drove up ozone and fine-particle pollution across the state.
Ozone is a tough pollutant to control because it is not emitted directly. Instead, it forms when emissions from cars, trucks, factories and other sources react in the heat and sunlight. And regulators and scientists have long predicted a lull in the pace of smog reduction, with diminishing returns as more of the obvious sources of pollution are slashed.
To continue reducing ozone, regulations must balance reductions of two main smog-forming pollutants: Combustion gases called nitrogen oxides and chemical vapors and solvents called volatile organic compounds.
Cut them in the wrong proportion and you may see no ozone reduction at all or even an increase, said Michael Kleeman, professor of civil and environmental engineering at UC Davis.
Kleeman and other researchers in a 2015 study analyzed air from Los Angeles and found it rich with nitrogen oxides, suggesting future controls need more balance. But he thinks it's unlikely there are large amounts of unaccounted-for emissions driving the increase in ozone.
Environmental advocates, however, point to recent studies that have found officials underestimating smog-forming emissions from some of the region's largest sources of air pollution, including diesel trucks and oil refineries.
Industry groups raise the opposite concern: That the air district has overestimated the emissions reductions needed to clean ozone to health standards, and could be forcing them to spend more than necessary on cleaner vehicles and equipment. The Truck and Engine Manufacturers Assn. wants the air district to examine its emissions and air quality modeling, asking officials to convene a special panel to evaluate why progress in reducing ozone pollution has slowed.
Regulators say cleaning the air to federal standards will require a massive transformation of California's transportation sector, which releases most of the state's pollution. Gas and diesel engines must be replaced with zero- and near-zero-emission battery electric and hydrogen fuel cell engines and other emerging technologies.
To meet federal ozone-reduction deadlines in 2023 and 2031, an air district smog cleanup plan adopted in March relies on raising $1 billion a year in cash incentives it says are needed to pay for lower-polluting vehicles — more than a tenfold increase from what it has spent in the past. While air district officials have secured more funding in recent legislation by state lawmakers, they admit that so far, they are falling far short.