In 1990, Beate Ritz was pregnant and living below a busy highway in smog-coated Los Angeles. When she gave birth, her son was surprisingly small. She suspected the vehicular exhaust from the I-10. But there was no proof. Ritz, an epidemiologist, made it her mission to find out whether exposure to air pollution during pregnancy could harm the baby. Her studies have revealed that underweight babies, premature births, and extremely premature births were more common among women who live in high traffic areas.

BOOK REVIEW — “Choked: Life and Breath in the Age of Air Pollution,” by Beth Gardiner (University of Chicago Press, 312 pages).

But air pollution is a slippery foe. Ritz still cannot say for certain whether her own son’s small size was due to the air she breathed. It can be nearly impossible to directly tie anyone’s particular illness to exposure to bad air, any more than to bad genes or lifestyle choices.

“[I]nvisibility is a strange feature of this crisis,” writes Beth Gardiner in “Choked: Life and Breath in the Age of Air Pollution.” Pollution’s victims, counted in aggregate and understood only through statistical analysis, are rendered faceless. As one Los Angeles environmentalist tells her, “You see one person run over on the street and you’ll never forget it.” But the thousands of deaths from air pollution “will never even faze you.”

Air pollution is the biggest environmental threat of our times. The latest State of Global Air report, released on April 2, finds that toxic air contributed to the deaths of nearly 5 million people in 2017. More than 90 percent of the world’s population was exposed that year to unsafe levels of PM2.5, particles 30 times smaller than the width of an average human hair. That’s so small that the particles can easily move into the bloodstream, organs, and even the brain. Exposure to toxic air has been linked to a range of diseases, from heart disease to diabetes to pneumonia.

Gardiner, an accomplished reporter, travels the rich and poor worlds, armed with an air mask, to understand the causes and effects of this silent killer, as well as the solutions. Although pollution respects no boundaries, people living closest to local hotspots usually suffer the most.

“It’s a dynamic playing out across the country and around the world,” writes Gardiner, “familiar to those who live beside big garbage transfer stations in the South Bronx, to Chicagoans who fought to force huge stockpiles of dusty, toxic petcoke (a byproduct of oil refining) out of their neighborhoods, to Londoners whose housing budgets consign them to the city’s most traffic-choked corners.”

She meets a family in New Delhi who live on a highway median, surrounded by lanes on three sides and, above, by an overpass. The children have a cough and a cold. As they fall asleep, oblivious to the traffic, the noise, the fumes, their father asks, “Where should we go?”

She talks over the phone to a family in Malawi who cook with charcoal on a three-stone fire. Pneumonia, often caused by indoor cooking fires, is the biggest killer of children under five in the nation. In London, where Gardiner lives, she worries for her 8-year-old daughter, who must breathe in diesel fumes every day on the walk to school.

Most governments are responding to the threat sluggishly, she reports. Take London, where nearly 10,000 people die every year from air pollution. Diesel cars fill the streets, spewing toxic particles and nitrogen oxides (NOx). The government pushed people to buy the cars beginning in the 2000s despite early warnings from scientists that diesel fumes can have serious health impacts. By 2012, more than half the cars in the United Kingdom were diesel. Other European nations, too, embraced the fuel in the belief that the exhaust could be cleaned up by catalytic converters.

Then, in 2014, Dieselgate broke. Volkswagen admitted that over a decade, it had installed software in 11 million cars in Europe and 600,000 cars in the United States that turned on pollution controls only during emissions tests. When the cars were on the road, they emitted NOx at up to 40 times the legal limit.

The galling thing, however, is that the violation was not restricted to Volkswagen; almost every diesel car manufacturer was violating norms. Regulators in some nations such as Germany had known, Gardiner writes, but enforcement essentially amounted to a shrug. “What I understand now is that the people we entrusted with the power to protect us essentially decided not to bother,” she writes. “I still can’t quite fathom it, and I want to understand how this gap between rules on paper and reality in the world could have grown so wide.”

Here, she finds an undercurrent tying the poor and rich worlds together. When politicians balance business and national interests against those of the environment and public health, the former tend to win. The long-term nature of the crisis allows authorities to prioritize more immediate concerns. In Europe, governments wanted to help automakers.

In India, where 270 million people live in poverty, the government wants to raise living standards and therefore promotes industry and infrastructure development. This leaves environmental protection as an afterthought. In Poland, where coal is considered “black gold” and miners are heroes, the nation still derives 85 percent of its power and 40 percent of its heating from the cheap fuel. “When European nations meet to discuss climate or air quality, to negotiate rules or deals that might help slow the planet’s warming or ease its people’s breathing,” Gardiner writes, “Poland is the eternal obstacle, coal’s staunchest defender, delaying and diluting any effort to hamper its use of that precious black gold.” But these choices come at an immense cost. In Poland alone, air pollution kills more than 45,000 people every year and costs the nation more than $100 billion annually, she writes.

Many accounts of pollution are harrowing in their scope and detail, but “Choked” is at heart an optimistic book. Gardiner insists that relief is immediately possible if governments prioritize health over more material concerns. In one gripping chapter, she points to the United States, which had toxic air in the 1960s. Cleveland’s Cuyahoga River had caught on fire and soot coated the lungs of Washingtonians living near a coal-fired power plant. The public demanded action, and Congress passed the Clean Air Act, which has withstood challenges by multiple administrations. It empowered the Environment Protection Agency to enforce safe limits, and allowed citizens to sue the government if air quality did not meet standards. Particulate pollution has since plunged and air quality continues to improve.

Gardiner also travels to China, which is presently undergoing its own clean air revolution, catalyzed by a documentary, “Under the Dome,” by the journalist Chai Jing. The video introduced millions to the hazards of PM2.5 pollution. As people voiced their discontent on social media, Jing mysteriously disappeared — and Gardiner’s attempts to find her end with a vague assurance from an environmentalist that Jing is fine. Since then, government has taken action, beginning with a commitment to transparency. Nearly 400 cities now post accurate PM2.5 readings, and the nation has a clean air action plan. “The moment you decide to disclose that, to give people the truth, then there’s no way for you to backtrack,” an environmentalist tells Gardiner. “It’s one way, you have to move forward.”

Gardiner next stops in Berlin, which she presents as a vehicular near-utopia that is mostly free of the noxious fumes that plague London or Los Angeles. Any destination is just 20 minutes away by bike, train, or walk. Berlin is what London should aspire to be, and it got there because its policymakers decided early on that its streets should belong to humans more than cars, an air pollution expert tells Gardiner.

Some nations are taking smalls steps in that direction. Many European countries are discouraging people from buying diesel and fuel taxes are rising across the continent, according to a Bloomberg analysis. Even more encouraging is the rise of the electric vehicles, fueled by China’s urgent desire to get millions of clean cars on the road.

Gardiner’s comprehensive reportage suggests it is entirely possible to tackle air pollution if only governments would prioritize the problem over more short-term concerns. Some technology is already available, people are ready and clamoring for their birthright, and some nations have illuminated the path. It now befits the rest of the world to follow and protect the health of millions.

Gayathri Vaidyanathan is an independent journalist writing about the environment, science, and society. Her work has been featured in The Washington Post, The New York Times, Discover Magazine, and Nature, among other publications.