Scientists say breathing the heavily polluted air in Mexico City these days is like smoking somewhere between a quarter- and a half-pack of cigarettes a day.

But that has not stopped Oscar Chong from going out for his daily workout, despite four days of warnings from the authorities to avoid strenuous physical activity outside.

“I’m addicted to exercise. If I don’t work out on a daily basis, I don’t feel well. It actually helps release my creativity, among many other things. If I just stayed home, I’d be staring at the walls, staring at my computer screen, and ideas are never born that way,” Chong, a graphic designer, told AFP.

The trim 51-year-old was taking a break from his interval workout in the capital’s largest park, the Bosque de Chapultepec — which the authorities actually closed at one point this week, to hammer home the message that running or cycling in the middle of an air pollution alert was not a good idea.

The sprawling city — a metropolitan area of more than 20 million people — has been blanketed in a thick cloud of smog since last weekend.

Authorities blame the problem on dozens of wildfires that have broken out across central Mexico in recent weeks, and the lack of wind or rain to disperse the resulting particles.

However, experts agree the city’s chronic pollution problems are also at fault.

Mexico City is prone to air pollution, both because of the mountains that surround it — trapping smog overhead — and its more than five million cars.

But the wildfires have undoubtedly made matters worse. They have sent the levels of PM2.5 soaring — tiny particles produced by any fire that are the deadliest air pollutant.

Authorities declared a pollution alert from Tuesday to Friday, after the micro-particle level hit 158 micrograms per cubic meter.

That is the equivalent of smoking more than seven cigarettes a day, according to a widely cited study by US doctors Richard and Elizabeth Muller.

On Friday, the level fell slightly, leading the authorities to call off the alert. But breathing the air was still equivalent to smoking nearly five cigarettes a day, according to the 2015 study, which compared deaths from air pollution and smoking.

The “goal of this calculation is to help give people an appreciation for the health effects of air pollution,” the Mullers wrote.

“Of course, unlike cigarette smoking, the pollution reaches every age group.”

‘Feels horrible’

The gray cloud of smog has scrambled people’s routines in the sprawling mega-city.

Officials are urging residents to avoid physical activity outdoors, and children, the elderly and those with respiratory illnesses to remain inside.

They have canceled school and sporting events. The football league moved a key semi-final match to Queretaro, some 200 kilometers (125 miles) to the northwest.

Many residents who can afford it have decided to do the same, skipping town until the pollution dies down — though many traditional getaway spots outside the city are polluted, too.

That includes the picturesque colonial city of Puebla, 135 kilometers to the southeast, which is dealing with an extra dose of pollution thanks to the nearby Popocatepetl volcano, which has been spewing ash into the sky.

Other residents have little choice but to ride out the smog, which stings many people’s eyes and throats.

“I’ve been trying not to go out. It smells like something burned,” Nicte Munoz, 38, said from behind a surgical mask on her way to the environmental organization where she works.

“It’s not at all good for our health. It feels horrible when you’re going up the stairs and suddenly you can’t walk or breathe,” said Diana Mariscal, 21, a communications student from the central city of Pachuca who was visiting for the weekend.

Authorities have shut down large construction sites, restricted the use of older vehicles and ordered certain polluting industries to cut emissions by 30 to 40 percent. They have even shut down some of the city’s beloved street-food stands to reduce smoke.

But Mayor Claudia Sheinbaum and President Andres Manuel Lopez Obrador — close allies in the leftist ruling party Morena — have faced criticism over the government’s slow reaction.

And none of the authorities’ anti-pollution measures amount to anything if they are not enforced, underlined Chong.

“Take the restrictions on older cars, for example,” he said.

“The (emissions) verification centers are full of corruption, and always have been. There may be a system designed to attack the pollution problem, but the reality is, it’s not. Pollution just continues, one way or another.”