Coal combustion, already one of the main sources of pollution, has also been found to release nano particles of titanium that are potentially harmful to humans and other life forms, researchers say.

US-based scientists accidentally discovered the tiny traces of the metal – in the form of titanium suboxides measuring in the billionths of a metre – when studying a coal ash spill in the Dan River in North Carolina in 2014, Science Daily reported earlier this year.

The substance had previously been considered rare, located in mudstones in a small area of western Greenland, moon rocks and in some meteorites.

It turns out, however, that forms of titanium oxides "are essentially a ubiquitous accessory phase in all coals worldwide", the researchers said in a paper published in Nature Communications.

Before electron microscopes became widely used, detection of so-called incidental nanomaterials had largely gone unnoticed or "were without concern", the scientists said.

"Now, with the development of environmental nanoscience and technology, this is more often no longer the case."

The authors of the paper emphasised that impacts of the titanium suboxides still needed to be "thoroughly tested for their toxicity in the human lung", but leading health experts say there is reason for concern.

Lidia Morawska, a leading researcher into nano particles at the Queensland University of Technology, says there is an increasing body of work showing the epidemiological impacts of the ultrafine particles.

Just as lead can have detrimental effects when it enters the bloodstream, other substances such as titanium could be expected to impact health.

"These are good reasons to expect they will cause impacts," said Professor Morawska, who is also co-chair of a World Health Organisation committee preparing to update global guidelines on ultrafine particles. "And these are good reasons to do something about it."

"These are ones that can get into the brain...or get into the liver, the kidneys, and pass through the placenta," Dean Schraufnagel, Executive Director of the Forum of International Respiratory Societies, said.

The Nature paper showed "there are some bad actors in these particles", said Professor Schraufnagel, during a Sydney visit to address the Thoracic Society of Australia and New Zealand.

These include known harmful materials such as hydrochloric acid and arsenic, which appear to be able to "hitchhike" on the nanoparticles, he said.

The fact that they are so small means they can stay airborne longer and spread far beyond the original sources. Diesel use is also likely to result in production of such nanoparticles, Professor Schraufnagel said.

Coal-fired power stations are known to be a major source of pollutants, including particulates of 2.5 micrometres (PM2.5) in size or smaller. Such particulates can be inhaled and enter the blood stream, causing pulmonary or cardiac diseases and leading to premature deaths.

"Nanoparticles are a part of particulate pollution and we should reduce exposure to that," Geoff Morgan, an associate professor of public health at Sydney University, said.

There are "lots of long-term reasons to take action to reduce carbon pollution" such as curbing climate change, Professor Morgan said. "But there are also many short-term reasons to address pollution."

James Whelan, a researcher with Environmental Justice Australia, said the research presented "a compelling argument to accelerate Australia's closure of coal-fired power stations, and to significantly strengthen our emission controls".

"Australia has one of the oldest, most toxic fleets of power stations in the world," James Whelan, a researcher and community organiser with Environmental Justice Australia, said. "None of our power stations are equipped to control for nanoparticle emissions."

A spokeswoman for the NSW Environment Protection Authority said the agency had "a comprehensive and robust air quality management framework".

"Environment protection licences include limits on the emission of particles," the spokeswoman said, while not specifying what standards it sets for nanoparticle emissions.

Separately, the EPA said it is still finalising a review of the state's coal-fired power stations prompted by a Fairfax Media report in May that at least one plant – the Bayswater facility now owned by AGL – had deliberately diverted less dirty coal to units being monitored for pollution.

So far, the review had "found widespread compliance with regulatory conditions," the EPA spokeswoman said.