Radioactive radon gas (222Rn) enters an average building at a rate of
4.5 picocuries per second per square meter of foundation area. Consider
a house with a foundation of 500 square meters and a volume of 1000 cubic
meters. Assume the house if well designed for energy conservation so that
the ventilation rate is low and only one tenth of the air in the house is
exchanged with outdoor air every hour. What will be the average steady state
concentration of 222Rn (in Ci/cubic meter) in the house?