(1 of 4) The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite acquired this pair of images of northeastern China on January 14 (smoggy) and January 3 (clear), 2013. Beijing is located at upper center, Tianjin at lower right, the image is approximately 250km wide. The first image shows extensive haze, low clouds, and fog over the region. The brightest areas tend to be clouds or fog, which have a tinge of gray or yellow from the air pollution. (Click image to see transition). The second image, from two weeks earlier, shows clearer air and snow cover. At the time of the smoggy image, the air quality index (AQI) in Beijing was 341. An AQI above 300 is considered hazardous to all humans. [click image to view transition - javascript required] #

(2 of 4) A before/after pair of photographs shows the Dongbianmen Gate, part of Beijing's ancient city wall, taken January 12, 2013, and (click to fade) January 16, 2013, in central Beijing. [click image to view transition - javascript required] #

(3 of 4) A before/after pair of photographs shows workmen atop a building under construction on a hazy morning and then in the afternoon (click to fade) in Beijing, on January 10, 2012. The Beijing city government will soon release the results of stricter air pollution standards, Chinese media reported on Friday, following a public outcry that authorities are understating the extent of smog that often shrouds the capital. [click image to view transition - javascript required] #

(4 of 4) This combination of photos shows the Beijing skyline during severe pollution on January 14, 2013, and the same view (click to fade) taken during clear weather on February 4, 2012. [click image to view transition - javascript required] #

This picture taken on January 9, 2013 shows a man standing on a boat as a haze of pollution filters the setting sun in the harbor in Hong Kong. Hong Kong is poised to get tough on thousands of ships burning dirty fuels that have turned the once-fragrant harbor into a city often covered in smog with air pollution killing over 3,000 people yearly. #

Heavy smog hangs over a road in Qingdao, east China's Shandong province on January 29, 2013. Residents across northern China battled through choking pollution on January 29, as air quality levels rose above index limits in Beijing amid warnings that the smog may not clear until January 31. #

Smoke billows from the chimneys of a power plant during sunrise in Jiaxing, Zhejiang province, on December 23, 2011. China will introduce stricter air pollution standards next year to monitor tiny floating particles of pollution in Beijing and other big cities but may not start releasing the results to the public until 2016, state media said. #

A man covers his mouth with his hand as he rides in heavy smog in Hefei, Anhui province, on January 14, 2013. Shares in a Chinese face mask manufacturer soared on January 15 as investors looked for opportunities to cash in on the severe air pollution that has blanketed large swaths of China. #

The Oriental Pearl Tower, on a hazy day in Shanghai, on January 21, 2013. Beijing is to unveil unprecedented new rules governing how China's capital reacts to hazardous air pollution, the official Xinhua news agency said, as deteriorating air quality threatens to become a rallying point for wider political dissatisfaction. #

Smog at Beijing airport at noon local time, earlier this month. Photographer John Williamson took this shot before boarding his plane, one of the lucky few to get out, as some 800 flights canceled that day. Williamson: "You literally can see the smog inside a large enough building. The airport terminal, the hotel lobby are large enough that you can no longer see clearly across the room." #

Students do exercises during a class break inside a school building on a hazy day in Jinan, Shandong province, on January 14, 2013. City residents were advised to stay indoors to avoid the heavily polluted air, Xinhua News Agency reported. #

Pollution readings from the US Embassy (upper) and the local government shows hazardous levels of air pollution in Beijing on January 23, 2013. At the height of recent pollution, Beijing authorities said readings for PM2.5 -- particles small enough deeply to penetrate the lungs -- hit 993 micrograms per cubic metre, almost 40 times the World Health Organization's safe limit. #

Two men walk on a frozen lake during severe pollution on January 30, 2013 in Beijing, China. The haze choking many Chinese cities covers a total area of 1.3 million square kilometers, the China's Ministry of Environmental Protection said. #

Severe pollution clouds the Beijing skyline on January 12, 2013. Air quality data released via the US embassy twitter feed recorded air quality index levels so hazardous that they were classed as 'Beyond Index'. Just after midday the particle matter (PM) 2.5 figure was 519 on a scale that stops at 500 and advises against all outdoor activity. #

Cans of fresh air are seen piled up before being given away by Chinese multimillionaire Chen Guangbiao, near a street on a hazy day in central Beijing, on January 30, 2013. China's recent streak of foul air has rekindled a tongue-in-cheek campaign by a Chinese multimillionaire with a streak of showmanship who is raising the alarm by selling canned fresh air. Chen Guangbiao, who made his fortune in the recycling business and is a high-profile philanthropist, on Wednesday handed out soda pop-sized cans of air, purportedly from far-flung and pristine regions of China, from Xinjiang in the far northwest to Taiwan off China's southeast coast. #