Households have been warned to be on guard against a natural gas that can cause lung cancer.
Radon, which is produced from tiny amounts of uranium in rock and soil, is believed to lead to more than 1000 deaths across Britain annually.
When testing kits were sent to some people across Aberdeenshire and the Highlands in 2011, higher-than-normal levels of the gas were detected in more than 800 properties.
Concentrated pockets were found particularly along Deeside in the north-east.
Neil McColl, of the Health Protection Agency's centre for radiation, chemical and environmental hazards, said: "We fully understand that when some people received their radon results they may have been surprised or even a little concerned because having a letter drop through the door telling you that your family is exposed to radiation could be unsettling for some people.
"Sadly, ignoring radon isn't a safe option. Failing to deal with high radon levels just increases your exposure and the risk to the health of you and your family.
"We are here to help and want to say to those who, for whatever reason, missed us when we ran local events that we're only a phone call or an email away."
Radon produces a radioactive dust in the air, the agency said.
The dust is trapped in airways and emits radiation that damages the inside of lungs.
Public Health Minister Michael Matheson said: "We are encouraging everyone who has received high radon results to come forward and find out more about radon and what they can do to their homes to lower levels.
"Radon cannot be detected by sight or smell but we now know that hundreds of homes across the north of Scotland have higher-than-safe levels. Failing to deal with high radon levels increases your exposure to it and in the long term this can be a health risk."
Householders wanting more information can call 01235 822 622 or email radon@hpa.org.uk.
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