Thick smog hovers over Beijing's Tiananmen Square as seen in this picture. Even the gigantic portrait of the country's founder Mao Zedong is covered by fog. Experts say that people living in the cities breathe in harmful gases.

In Beijing alone, there are more than 5.4 million registered vehicles, whereas in Shanghai there are about 2.7 million cars. The increase in traffic over the years has been blamed for the poor air quality. According to statistics provided by Greenpeace, more than 90 of China's 190 major cities exceed the limits set by by the authorities.

These students perform their morning exercises in the school's hallway. The authorities recommend both young and elderly people not to train outside, as this might seriously damage their health.

Anyone in China who wants to indulge in some physical activity must be creative. This runner wears a gas mask while jogging in the Chinese capital's Olympic Park.

The old power plants and cement factories in northern China emit pollutants that compound the problem. Seven of China's ten most polluted cities are located in Hebei province. In a trip to China in April 2014, Germany's Foreign Minister Frank-Walter Steinmeier campaigned for the use of environment-friendly technologies.

Chinese film director Jia Zhangke made a short film about air pollution in China in 2013. Designed to raise awareness, the movie depicts how Chinese people deal with smog on a daily basis. Jia's father died from lung cancer nine years ago in the coal-rich Shanxi province.

"When I woke up this morning and opened the window I thought I was blind," wrote a Chinese citizen on Weibo, the Chinese micro-blogging website, "but then I realized it was smog."

The authorities managed to bring down pollution levels in Beijing before the APEC Summit in November 2014, as seen in this picture of the conference venue. The officials shut down construction sites and factories in the area, and traffic was artificially reduced. Many in China dubbed the summit "APEC Blue" for its environmental success.

The worst nuclear disaster of all time, the explosion at Chernobyl nuclear plant in Ukraine released massive amounts of radiation into the atmosphere. Areas close to the plant - in Ukraine, Belarus and Russia - were heavily contaminated. Heightened levels of radiation were also measured across most of Europe. The "exclusion zone" around Chernobyl remains off-limits to human habitation today.

After a magnitude-9 earthquake and consequent tsunami, three nuclear reactors at Fukushima power plant in Japan went into meltdown in March 2011. There were also four hydrogen explosions. The accident released 500 times as much radioactive cesium-137 as the nuclear bomb dropped on Hiroshima in 1945. The clean-up is expected to take decades.

After Chernobyl, thousands of people developed cancer. In Japan too, the heavily contaminated region of Fukushima, where 200,000 people lost their homes, saw cases of the disease escalate. The number of children with thyroid cancer there is 20 times higher than other regions.

Chernobyl fueled public opposition to nuclear power, particularly in Europe. The same happened after Fukushima. Before the Japanese disaster, the country relied on nuclear for 30 percent of its power. That has fallen to 1 percent. The government wants to continue producing nuclear power and plans to reinstall some reactors. But affected regions have successfully pushed back those plans.

Today, the nuclear power sector is deep in economic crisis. In Japan, the United States and France, nuclear power plants run at a loss, and construction projects for new reactors have been postponed.

France had high hopes for its newest nuclear reactors - called pressurized water reactors (PWRs). This technology was supposed to be safe, and the Flamanville power plant was due to be switched on in 2012. Due to security issues, that's been pushed back to 2018 at the earliest. The project will cost more than 10 billion euros - three times the original budget.

For years, the UK has been planning to build two new PWR reactors at Hinkley Point. Costs are estimated at 33 billion euros and groundbreaking is slated for 2019. But doubts are growing over its economic viability. The electricity it produces will be much pricier than solar or wind power, and will need subsidies to compete in the market.

Nuclear power plants used to be lucrative. But now, many are old and frail. Repair costs often mean they cannot turn a profit. Swiss energy corporation Alpiq recently tried to give away two of its old plants, 33 and 38 years old, to French energy company EDF - which declined the offer.

Three decades ago, the Chernobyl disaster galvanized Germany's anti-nuclear movement, which is often cited as the roots of the country's energy transition. In 2002, Germany passed a law that would have seen the last reactor shut down in 2022. The plan was later scrapped by Angela Merkel's government. But after Fukushima, Merkel quickly reversed her decision and the phase-out was back on track.

So far, nine of Germany's reactors have gone offline, with eight more to follow by 2022. To finance the costs of nuclear waste disposal, plant operators must pay 23.6 billion euros into a federal fund. The operators themselves are responsible for the similarly costly process of dismantling the plants, which will take decades to complete.

Across the EU and Switzerland 132 nuclear reactors are still online. They were designed to operate for 30 to 35 years - their average age is now 32 years. Malfunctions and security issues are frequently detected and protestors are increasingly calling for plants to be shut down.

No new nuclear power plants have been built in the EU, Japan or Russia since the Fukushima disaster in 2011. China remains committed to nuclear, partly to replace coal-based power. But the country is also upping investment in wind and solar.