Measurements of indoor radon concentrations in the village Umhausen (2
600 inhabitants, Otztal valley, Tyrol, Austria) revealed unusually hig
h indoor radon concentrations up to 274,000 Bq m-3. The medians measur
ed on the basements were 3750 Bq m-3 in winter and 361 Bq m-3 in summe
r, those on the ground floors were 1180 Bq m-3 and 210 Bq m-3, respect
ively. Seventy-one per cent of the houses showed basement radon concen
trations above the Austrian action level of 400 Bq m-3 in winter, 33%
in summer. There are indications that the high radon concentrations ar
e due to a giant rock slide about 8700 years ago. The unusually high r
adon concentrations in Umhausen coincide with a statistically signific
ant increase in lung cancer mortality. For the period 1970-1991 the ag
e and sex standardized mortality rate is 3.85 (95% confidence interval
: 2.9 to 5.1). The control population is the total population of Tyrol
(630,000 inhabitants).