By Paul Rincon
Science reporter, BBC News, Houston
Scientists are investigating the possible threat posed to astronauts by inhaling lunar dust.
Lunar dust brought back to Earth from the Apollo 17 mission
A study suggests the smallest particles in lunar dust might be toxic, if comparisons with dust inhalation cases on Earth apply.
Teams hope to carry out experiments on mice to determine whether this is the case or not.
Nasa has set up a working group to look into the matter ahead of its planned return to the Moon by 2020.
A team at the University of Tennessee (UT) in Knoxville is also looking at ways of using magnets to filter dust from the living environments of lunar bases and spacecraft.
The health effects of inhaling lunar dust have been recognised since Nasa's Apollo missions.
Astronaut Harrison H (Jack) Schmitt, the last man to step on to the Moon in Apollo 17, complained of "lunar dust hay fever" when his dirty space suit contaminated the habitation module after an energetic foray on the lunar surface.
The US space agency (Nasa) is now keen to assess the effects of more prolonged exposure and to address the problem before humans are sent back to the Moon in just over a decade.
Details of the work were presented to the Lunar and Planetary Science Conference in Houston, Texas.
Nasa's Lunar Airborne Dust Toxicity Advisory Group (LADTAG), which includes medical doctors as well as scientists from UT, have been working to characterise the dust's properties.
Lunar dust will be tested before humans set foot on the Moon again
"I've been working on lunar samples for 35 years and I have looked at fractions down to a few microns (millionths of a metre), but never anything less," said Professor Larry Taylor, director of the Planetary Geosciences Institute at Tennessee.
"The medical doctors are interested in things that are less than about three microns.
"So we did some particle size determinations and discovered that a very large portion of lunar soil is potentially dangerous, approximately 1-3% of the total soil by weight."
Most particles in lunar soil should get coughed up, or moved out of the lungs by specialised hairs called cilli. But any particles smaller than about 2.5 microns will stick in lung tissue.
When dust is deposited in the lungs, inflammation occurs. Ultimately, scar tissue called a fibroid grows around the particle. This scar tissue replaces cells which facilitate the exchange of oxygen for carbon dioxide (CO2) in the lungs.
The magnetic properties of lunar dust could be useful
This is the process at work in the condition silicosis - an occupational hazard in mining, quarrying and foundry work - and asbestosis, which results from inhaling asbestos fibres. It also occurs in bronchitis and as a consequence of smoking.
"If you took a healthy pair of adult lungs and smeared them out, they would cover a football field," Professor Taylor told BBC News.
"Once you are down to the size of a square table's worth of surface area in your lungs that is useable; you are just about dead."
The team at the University of Tennessee has shown that about one percent by weight of lunar soil comprises particles less than one micron in size. A smaller - but still significant - fraction is less than 100 nanometres (billionths of a metre) in size.
They determined that most of the fine particles in lunar dust are composed of glass formed through the impact of micrometeorites on the surface of the Moon. But the glass also contains metallic iron grains, much like that in a carpenter's nail and measuring just 10-20 nanometres in size.
Machines could be sent to the Moon to "pave" its surface
These grains, called "nano-phase iron", are so small that, if inhaled, some would pass directly from the lungs into the blood circulation.
Once in the blood, the iron could "de-energise" the haemoglobin molecule which carries oxygen to the body's tissues. If enough gets dissolved in the blood, it could produce effects similar to carbon monoxide poisoning.
However, exactly how much is required for this to happen remains an open question.
In addition, when some fine dust particles are examined under the microscope, they can be seen to be filled with holes - like Swiss cheese.
These vesicles give them a much larger surface area to react with the lung tissue, says Dr Yang Liu, a postdoctoral research associate at the University of Tennessee.
"If you have a solid particle of dust and add vesicles, you can calculate how much the reactive surface area is increased. Sometimes you can get increases of up to a factor of five," she told BBC News.
"With jagged particles, because of the way they follow the path of the air, there's a lower chance of them impacting the sinus walls at the back of the throat - which is the body's defence mechanism for keeping particles out of the lungs," said Dr Benjamin Eimer, another postdoctoral researcher at the University of Tennessee.
Previous in vitro experiments with rodents found little effect from the inhalation of samples of lunar soil of average grain size. But Larry Taylor says that new experiments need to concentrate on the effects of breathing in the fine dust particles.
However, the nano-phase iron could also be the key to mitigating the hazard, because it imparts magnetic properties to the dust.
Virtually all particles smaller than about 50 microns are attracted by a simple hand-held magnet.
To demonstrate, Professor Taylor brought with him a vial of lunar dust returned from the Moon in the 1969 Apollo 11 mission. Sure enough, the dust was attracted to the magnet as he moved it around the plastic container.
"I discovered that if you put lunar soil in your microwave oven, next to your tea, it will melt at 1,200C before your tea boils - which is a magical thing," he said.
This property is almost entirely due to a coupling effect between the microwaves and the nano-phase iron in the dust.
The molten dust from the microwave treatment hardens into solid glass, which has given rise to the idea that robots could be sent ahead of human missions to "pave" landing pads and roads by firing microwaves at the lunar soil.
Professor Taylor has come up with a concept for a wheeled vehicle, much like an ice-rink resurfacer, that could perform the task, and has developed a prototype microwave device that could be carried to the Moon.
Paving the Moon would prevent dust being kicked up by the manned spacecraft and vehicles which would follow to the surface, but some of the finest dust may be "levitated" electro-statically above the surface. This is relevant not only for the health of the astronauts, but also for astronomy on the Moon.
"If they are going to do any work on the Moon, they don't want dust in the way, so there is a big effort to minimise it," Dr Eimer told BBC News.
He has been working on ways to filter dust out of habitation modules and vacuum it up from the surface with devices that use magnets to attract the lunar soil and dust.
One concept, called the Lunar Air Filter with a Permanent Magnet System (LAF-PMS), consists of many permanent magnets placed with their magnetic poles very close, creating a large field gradient that attracts lunar dust particles from the air.
In order to clean the filters out, small blocks of iron are moved into the gaps between the magnets to close off the magnetic fields. With the filter turned off, the dust can be wiped away.
Another concept, called the Lunar Soil Magnetic Collector (LSMAC), comprises a series of wound magnetic coils arranged along a tube that could be turned on in sequence to effectively "suck up" lunar dust, giving the impression of the leaf-gathering machine used here on Earth.
"We will want to collect the soil to extract oxygen and hydrogen and perhaps to use for building materials. So we will have to collect massive amounts of lunar regolith. Our idea was to make something that could gather up the soil without creating a large dust cloud," said Dr Eimer.
(1) The heavy-lift Ares 5 rocket blasts off from Earth carrying a lunar lander and a "departure stage"
(2) Several days later, astronauts launch on an Ares 1 rocket inside their Orion vehicle (CEV)
(3) The Orion docks with the lander and departure stage in Earth orbit and then heads to the Moon
(4) Having done its job of boosting the Orion and lunar lander on their way, the departure stage is jettisoned
(5) At the Moon, the astronauts leave the Orion and enter the lander for the trip to the lunar surface
(6) After exploring the lunar landscape for seven days, the crew blasts off in a portion of the lander
(7) In Moon orbit, they re-join the waiting robot-minded Orion and begin the journey back to Earth
(8) On the way, the service component of the Orion is jettisoned. This leaves just the crew capsule to enter the atmosphere
(9) A heatshield protects the capsule; parachutes bring it down on dry land, probably in California