The high level of smog on Monday led authorities to close hundreds of factories and schools. Wang Zhao / AFP / Getty Images

Measurements of the air in Beijing recorded pollutants nearly 25 times higher than the standard set by the World Health Organization. Kevin Frayer / Getty Images

As world leaders discuss climate change in Paris, Beijing and northern China experienced the worst smog of the year Kevin Frayer / Getty Images

A man wears a mask as protection from pollution near the Forbidden City in Beijing, Dec. 1, 2015. Kevin Frayer / Getty Images

Schools in the Chinese capital kept students indoors and parents brought their kids to hospitals with breathing ailments Tuesday as Beijing grappled with extremely severe air pollution for the fifth straight day.

"It's the worst day so far this year," said Liu Feifie, a 36-year-old mother and Internet company employee. "I feel my throat totally congested with phlegm and it feels very itchy. But I'm more concerned about the health of my 7-year-old kid."

The pollution spike is a reminder of China's severe environmental challenges as President Xi Jinping joins other world leaders at the Paris climate conference. The hazardous air underscores the challenge facing the government as it battles pollution caused by the coal-burning power industry and will raise questions about its ability to clean up its economy at the talks in Paris.

Factories and construction sites were told to reduce work after the city government on Sunday issued its first orange alert — the second highest of four warning levels — in almost two years.

On Tuesday, Beijing schools were ordered to stop outdoor activities. A primary school in the city's Xicheng district sent a message to parents that classes were canceled Tuesday.

Outside a packed children's hospital in downtown Beijing, parents and grandparents complained about the smog's impact on small children and say the pollution has made their children vulnerable to illnesses such as throat infections and the flu.

"The government is supposed to be tackling the pollution, so we need to see the effects. If in a few years the situation does not change, we will consider leaving," said Yin Lina, who brought her 5-year-old daughter to the hospital with a stuffed nose.

Readings of the tiny poisonous PM2.5 particles reached into the high 600s micrograms per cubic meter through the capital, as compared with the World Health Organization safe level of 25. Some suburban neighborhoods logged levels up in the 900s on Monday. Such particles are especially damaging to lung tissue.

Visibility fell to several hundred yards, leaving buildings silhouetted in the haze. People complained of a smoky, pungent odor and many wore tight-fitting face masks.

"I felt like my lungs were blocked," said Xu Pengfei, a security guard at a downtown office building. "We have to stand in the open for many hours a day, and the pollution really affects us."

China's cities are among the world's dirtiest after three decades of economic growth that led to construction of hundreds of coal-fired power plants and the spread of automobile ownership.

Leaders have tightened emissions standards and are investing in solar, wind and other renewable energy as part of a “war” on pollution. But the country still depends on coal for more than 60 percent of its power.

Tests found coal burning to be to blame for the bulk of the latest pollution surge, the official Xinhua News Agency said, citing Zhang Dawei, head of the city's environmental monitoring center.

Power demand has soared due to unusually cold weather in November. For most of that month, persistent smog shrouded the capital.

Air quality worsened on Friday and deteriorated throughout the weekend. Authorities said they avoided issuing the highest-level alert because conditions were forecast to improve by Wednesday.

Conditions were worsened by cold air that trapped pollutants near the ground, according to Zhang, the environment official. He said pollution from surrounding areas also blew into the capital.

Outside Beijing, reduced visibility due to heavy fog prompted authorities to close 1,553 highway sections in central, eastern and southern China, the Transportation Ministry said on its website.

The Associated Press