Shanghai smog hits crisis point: Children ordered indoors and visibility limited to a few dozen metres with air pollution 24 times WHO safety levels Harmful PM 2.5 particles hits 602.5 micrograms per cubic meter World Health Organisation safety guideline sets limit of 25 micrograms Authorities order halt to construction in China's financial hub Children have been warned not to venture outside as Shanghai's hazardous smog levels reach crisis point. Authorities in China's financial hub also ordered a halt to construction work as the city suffers one of its worst bouts of air pollution in history. The dense and dangerous smog - which reached more than 24 times the World Health Organisation's safe limit - caused visibility in China's most populated city to fall to just a few dozen metres, obscuring its spectacular skyline. Grey day: A man stands on the Bund in front of the financial district of Pudong during a hazy day in Shanghai Clearing: The smog gradually lifted, allowing a glimpse at the world famous skyline during the day Clear: This is a similar view of the Shanghai skyline without the dangerous smog Authorities say the city's concentration of tiny, harmful PM 2.5 particles was 602.5 micrograms per cubic metre early Friday afternoon, an extremely hazardous level. That compares with the World Health Organization's safety guideline of 25 micrograms. The Shanghai environmental authority today even took the drastic move of lowering air pollution standards in an attempt to reduce the number of alerts being issued. Smog: An electronic screen and buildings are seen amid heavy smog at the financial district of Pudong in Shanghai today Choking: A man wears a face mask while walking on the Bund in downtown Shanghai this morning Alerts had previously been activated when the concentration of PM2.5 reached 75 micrograms per cubic metre. Shanghai weather authorities could soon be given the all clear to use cloud seeding to generate rain. The aim is the showers will to purge the atmosphere of dangerous particles. It is part of the government's plan to invest 1.7 trillion yuan ($277 billion) in tackling air pollution. The technique involves firing rockets carrying a payload of silver iodide particles into the clouds, according to New Scientist. Ice crystals then form from super-cooled liquid water in the clouds, using the particles as nucleation points onto which the hazardous air particles latch and fall to earth. But now the warning will be triggered by Shanghai's Environmental Protection Bureau when the dangerous particle concentration hits 115 micrograms per cubic meter, according to a report by the China Daily. Experts blame Shanghai's pollution on coal emissions in the region combined with weather patterns that have left the city's air stagnant. The issue of air quality has now become a serious concern for China's leadership as it plays into popular resentment over political privilege and rising inequality in the world's second-largest economy. Health dangers from the severe pollution are also becoming more apparent. Smog in China's most populated city is being blamed for a fertility crisis as the incidence of low sperm counts among its men reaches record levels. Doctors in financial capital Shanghai say air and water pollution is a "major culprit" for the city's predicament and warn that action needs to be taken urgently. Gloom: Shanghai's skyline is almost invisible through the thick smog which hangs over the city "If we don't protect the environment now, mankind will face a worsening infertility predicament," Dr Li Zheng, a sperm expert at the city's Renji Hospital told the Shanghai Morning Post. Only a third of the semen at Shanghai's main sperm bank, run by Dr Li, currently meets World Health Organisation standards. And smog levels are rising. A 2012 study, coordinated by Dr Li, concluded that over the last ten years, worsening environmental conditions were keeping pace with the falling quality of sperm and aspermia, a condition that causes men to produce no semen at all. A year ago, China's infertility rate stood at 12.5 per cent of people of childbearing age, according to China state news agency Xinhua. Twenty years ago it was just three per cent. Grinding to a halt: A man wears a face mask while walking his pet dog on a street during a hazy day in downtown Shanghai as authorities stop major construction projects in the city Seeing through the smog: Aircraft are barely visible through thick smog on the tarmac of Hongqiao airport in Shanghai as severe pollution blankets the city on