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EFFECTS OF VENTILATION ON RADON CONCENTRATIONS IN NORWEGIAN KINDERGARTENS

A.Birovljev, T.Strand and A.Heiberg
Norwegian Radiation Protection Authority, P.O.Box 55, N-2345 Østerås, Norway

ABSTRACT
Ventilation systems are known to influence radon concentrations indoors considerably. However,
the initial results of the largest radon survey in 3660 Norwegian kindergartens conducted during
heating seasons from 1996 to 1998 showed marginal effect of such systems. Clarification of this
surprising result required a more detailed secondary investigation in a smaller group of
kindergartens with balanced ventilation systems operating under special ventilation regime.
Both the primary survey and the secondary investigation were conducted using CR-39 etched track
detectors with minimum exposure time of two months. In the secondary investigation the
kindergartens, which in primary survey had measured radon concentrations above 200 Bq/m 3 , were
instructed to let ventilation systems operate continuously, 24 hours a day, instead of a normal,
energy saving regime (on - during daytime, off - during night). It was assumed that the average
radon concentration measured under 24 hour ventilation conditions will be very close to the average
radon levels during daytime, when ventilation is operating under normal, energy saving regime. It is
the daytime exposure to radon that is related to the dose the children and staff receive.
The primary survey of 3660 kindergartens gave nearly a log-normal distribution of radon
concentrations with arithmetic mean of 87.9 Bq/m 3 and geometric mean of 44.5 Bq/m 3 . The
fraction of kindergartens with radon concentrations above 200 Bq/m 3 was 9.2% and 2.7 % had
results above 400 Bq/m 3 . In the secondary investigation in 22 kindergartens, which ran their
ventilation systems 24 hours every day, the average reduction in radon concentration was 77%,
compared to 5% reduction in a control group that operated ventilation systems in normal regime.
Additional study of variations of radon concentration in a few high radon kindergartens using active
instruments helped determine optimal morning hours when ventilation systems should be switched
on to keep radon concentration at minimum during daytime.
The results open a wider questions on how radon measurements at work places should be conducted
and what results should be the bases for setting action limits for work places. From the health point
of view it is relevant to measure average radon concentration only in periods when the building
(especially if it has a ventilation system installed) is in use. From the house-construction perspective
it may be more appropriate to look at the total average.