SYDNEY, Australia (CNN) -- Most people looking out across the sky of a large city are aware that breathing in that hazy, gritty cocktail of suspended pollutants simply cannot be good for the health.
The effect of high concentrations of suspended particles in the atmosphere on respiratory health -- triggering breathing disorders and lung diseases -- is well documented.
But what they may not be aware of is that one of the greatest threats to their well being could be from air pollutants that they cannot see: Ultrafine particles so small many air quality monitors don't even pick them up.
These microscopic killers are emitted from vehicle exhausts and could be responsible, the World Health Organization estimates, for as many as 800,000 deaths a year.
This is not to dismiss the negative health impact of the larger, more visible particles in the air.
According to the World Bank, as many as 300,000 people in China die each year as the result of respiratory illnesses triggered by air pollution.
Rapid industrialization in China has resulted in the nation now holding the dubious distinction of having 16 of the world's 20 most polluted cities.
One province, Shanxi, has four of the top 10, including the world's worst: the city of Linfen.
A recent report from the World Bank singled out China and India for the rapid rise of greenhouse emissions in those countries. (Full story)
A study published last year in the Journal of Toxicology and Environmental Health (JTEH) finds the developing nations of Asia suffer about two-thirds of the health problems due to urban air pollution.
Even a less-industrialized Chinese city, such as Hong Kong, is suffering from an increase air pollution, much of it blowing in from the burgeoning factories on China's Pearl River delta.
The delta is the manufacturing heart of the new China, generating one-third of the nation's exports, but also up to 80 percent of Hong Kong's air pollution, brought to the city courtesy of the prevailing northerly winds.
Civic leaders fear the city's attraction as an international business center and tourist destination is being eroded by its neighbors' dirty habits. (Full story)
That's just one price to pay.
With air pollution from all sources responsible for 1.9 million premature deaths a year, according to the JTEH article -- more than those caused by tobacco smoke and that twice that of AIDS -- governments have the task ahead of them to balance the benefits of economic growth against the negative cost to public health.
What is most disturbing, is the increasing evidence of a link between ultrafine particle pollution and an incidence of heart disease.
According to an article in "Heart," the journal of the British Cardiac Society: "Epidemiology has clearly shown a link between increases in particulate air pollution and deaths and admissions caused by heart failure, myocardial infarction and arrhythmia."
While scientists have yet to fully explain exactly how the presence of the ultra-fine pollutants causes increased heart disease, "the association of air pollution with cardiac mortality and morbidity is beyond doubt," the journal says.
Many pollution researchers believe vehicle exhausts pump out microscopic specks of carbon which are coated with chemicals such as chromium, peroxide and hydrocarbons resulting from the burnt fuel.
These particles measure less than one-tenth of a micron across. They are so small that they can pass easily through the walls of a human lung and penetrate into the body's red blood cells.
From there, they wreak health havoc, penetrating the body's cells and disabling them. Recent laboratory studies suggest these ultrafine particles can be up to 50 times more damaging than bigger particles, possibly triggering heart attacks.
Diesel emissions are thought to be disproportionately responsible for pumping out ultra-fine particles, making people living or working close to major transport routes especially vulnerable.
So why aren't health authorities doing more to combat this threat?
One reason, according to Australian research scientist Melita Keywood, is the lack of hard data to support activists' claims.
The measurement of ultrafine particles in the atmosphere requires highly specialized equipment, which many authorities do not have.
Keywood, who works for the Marine and Atmospheric Research Division of Australia's key science research organization, CSIRO, says that where data has been collected authorities have acted.
Concerns for the young, elderly
In California, for example, schools and childcare facilities have been built away from busy highways as a result of evidence of high concentrations of ultrafine particles in those locations.
The lack of a well-documented causal link between the ultrafine particles and negative health consequences is also hindering health authorities.
"We still don't know what compounds it is that are causing the problems," Keywood told CNN. "There is still a lot of open work to do."
The link between air pollution and heart disease is particularly worrisome for the elderly.
An Australian medical study published in March this year explored the link between air pollution and the hospitalization of elderly people for cardiovascular disease in Australian and New Zealand cities.
The study concludes that "air pollution arising from common emissions ... have significant associations with adult cardiovascular hospital admissions -- especially in the elderly -- at air pollution concentrations below normal health guidelines."
With some research suggesting nearly 100 million North Americans are living in air that consistently exceeds the recommended safe limit for fine particles, the potential health cost is mind boggling.
CNN's Jill Dougherty and Stan Grant contributed to this report.