Tests are to be carried out on around 1000 homes amid fears they contain high levels of a radioactive gas that can lead to lung cancer.
Households in Danderhall, Loanhead and Mayfield will get letters telling them that free radon-detecting tests worth £50 each will take place.
The Health Protection Agency estimates the county has just under 1000 homes with a five per cent or greater risk of exceeding the radon “action level”.
Affected properties are to be sent two small plastic monitors, which will remain on site for three months before being analysed. Midlothian Council will decide on what action to take depending on the results.
Eibhlin McHugh, the council’s acting director of communities and wellbeing, said: “Radon is generally only harmful if it accumulates in homes and other places where people are present for extended periods.
“Being radioactive, it has the potential to cause cancer. However, the risks can be reduced by measures such as increasing under floor ventilation.
“This risk from radon is greatly increased where the individual affected smokes.”
Every building contains radon but the levels are usually low.
A colourless, odourless radioactive gas, radon is produced when uranium decays in certain types of rock, especially granite. Exposure to high levels of radon over extended periods leads to an increased risk of lung cancer and estimates suggest that around 2500 cases are caused by indoor radon each year in the UK.
Robert Hogg, of Mayfield and Easthouses Community Council, said: “I’m shocked to hear this. That this is something that can cause cancer and ultimately lead to death – how long has it been going on?
“We need to find out all the facts, exactly how many homes have been affected, where they are and exactly how long this has been going on.”
In Scotland, high levels of radon have been associated with Aberdeenshire, Highland and Orkney – but Midlothian had historically been considered free of problems.
But in 1995, engineer Anthony Gardner said miles of abandoned Lothian mines could be leaking radioactive radon gas into homes. He called for a full investigation to assess the scale of risk.
Loanhead councillor Russell Imrie said: “People who live in the affected areas will be quite rightly very concerned about this, and if it is connected to abandoned mines, we’re not just talking about Midlothian, we’re talking about the whole of Scotland.
“The only way to reassure people is to get a clearer idea of what’s going on.”
Residents who are not included in the testing programme, which is being paid for by the Scottish Government, can pay to have one conducted.
Mary Dempster, 66, a retired community education worker who has lived in Mayfield since 1969, said: “I would not be happy about anything that could have affected my health. It could really have an impact on this area because there are so many abandoned mines.”