Heavy smog suffocated northeast China for a fifth day Tuesday, with hundreds of flights cancelled and road and rail transport grinding to a halt under the low visibility conditions.

More than 20 cities have entered a state of red alert since Friday evening, implementing emergency measures aimed at cutting emissions and protecting public health from the toxic miasma.

Chinese cities are limiting the number of cars on roads and have temporarily shut down factories to cut down pollution during a national "red alert" for smog.

State media reported Monday that more than 700 companies stopped production in Beijing, and that traffic police were restricting drivers by monitoring their license plate numbers. Dozens of cities closed schools and took other emergency measures after a "red alert" was issued from Friday night to Wednesday for much of northern China.

The battle to retake Mosul from the Islamic State group is leaving a legacy of environmental damage and health risks that will pose dangers to people for years to come.

China's capital and other northern cities have banned half of all vehicles from city streets and ordered factories, schools and construction sites closed in response to a five-day smog red alert.

The emergency steps significantly reduced traffic in Beijing on Saturday, although it wasn't clear what effect it was having on air pollution. By midday, the capital was enveloped in a smothering layer of smog, with concentrations of microscopic PM2.5 — small, inhalable particles that can penetrate deep into the lungs and are considered a reliable gauge of air quality — rising to more than 10 times the level considered safe by the World Health Organization.

When the world triumphantly celebrated the signing of the landmark Paris climate pact last December, it was hard to imagine that only a year later it might face an existential threat.

Beijing issued its first air pollution red alert for 2016 Thursday, as choking smog is expected to cover the city and surrounding areas in north China during the next five days.

From 8 pm on Friday, half of Beijing's private cars will be ordered off the road, with an odd-even number plate system in force, according to a notice posted on the official website of the Beijing municipal government.

Environmentalists have hit out at organizers of the Miss Universe beauty pageant in the Philippines, calling them "irresponsible" for letting contestants swim with endangered whale sharks.

Animal rights defenders stripped down Sunday in the Spanish tourist hub of Barcelona to draw attention to the cruelty of the fur industry.

A pair of jetlagged sea otters arrived in France on Friday after a 9,000-kilometer (5,600-mile) flight from Alaska to their new home at a sea life park.

Leonardo DiCaprio met President-elect Donald Trump to discuss climate change and how renewable, clean energy could boost the economy by creating millions of new jobs, the actor's foundation said Thursday.

Terry Tamminen, chief executive of the Leonardo DiCaprio Foundation, said in a statement he and the Hollywood star presented the Republican property tycoon, his daughter Ivanka and other aides with a plan to unleash "a major economic revival" through investments in sustainable infrastructure.