GETTY•SCOTTKELLY•TWITTERAn image of earth's atmosphere from space and (inset) Scott Kelly
US astronaut Scott Kelly spoke of his fears as he prepared got leave the International Space Station (ISS) and head back to Earth.
In an interview about the condition of the planet as seen from the ISS, he said: "When you look at the atmosphere on the limb of the Earth, I wouldn't say it looks unhealthy, but it definitely looks very, very fragile and just kind of like this thin film, so it looks like something that we definitely need to take care of."
Mr Kelly has been known for keeping a keen eye on environmental issues from above and has often tweeted about pollution levels visible from space.
In one tweet he said: "This is a good example of the air pollution that exists across large parts of Asia."
SCOTTKELLY•TWITTEROne of Scott Kelly's stunning views he tweeted from the ISS
He is due to return to earth in March, but was asked this month by CNN to describe the Earth's condition if it were a human body.
He added: "There are definitely parts of Asia, Central America that when you look at them from space, you're always looking through a haze of pollution.
"As far as the atmosphere is concerned, and being able to see the surface, you know, I would say definitely those areas that I mentioned look kind of sick."
Mr Kelly's remarks will be of concern as the atmosphere is the main reason life evolved and stays living on Earth.
It is why we have oxygen to breathe, fends of ultraviolet radiation from the sun, keeps the temperature right, makes the weather system that creates our water cycle, and also provides protection from incoming objects such as meteorites which burn up within it.Scott Kelly tweeted this image of pollution over Asia
As far as the atmosphere is concerned, and being able to see the surface, you know, I would say definitely those areas that I mentioned look kind of sick.
And according to NASA, our atmosphere has been thinning as a result of carbon emissions.
In 2003 the agency noted the effects were already visible from space.
In a statement at the time, it said: "From a vantage point about 225 miles over the Earth, space station crew members photographed the crescent moon through the upper layers of Earth’s atmosphere.
"At the bottom of the image, a closed deck of clouds is probably at about three miles.
"The shades of blue grading to black are caused by the scatter of light as it strikes gas molecules of the very low density upper atmosphere.
"Models predict that emissions of carbon dioxide are causing the upper atmosphere to cool and contract, and therefore reduce the density of gases in the layer spanning from 60 to 400 miles above the surface—known as the thermosphere."
Studies by the Naval Research Laboratory show the density of the thermosphere has decreased more than 10 per cent over the last 40 plus years.