| dc.description.abstract | Exposures of human beings to the natural background radiation in Kenya are
assessed using measured activity concentrations of natural radionuc1ides and
dose conversion factors; The sampling locations for geological materials (soils
and rocks) and water span across different geological terrain in the country. But
they are largely restricted to the more densely populated areas, mainly in
Nairobi, Kiambu, Kwale, Mombasa, Machakos, Bungoma, and Tran Nzoia
districts. The activity concentrations in the samples are found to vary according
to the geological formations in the sampling locations. They are generally
higher in samples from basements and volcanic formations than in those from
sedimentary formations.
The dose conversion factors used in estimating external exposures to radiation
from gamma-ray emitters in soils and walls of buildings are calculated using
the point kernel integration method. For an outdoor point receptor in air at 1m
above the ground the dose conversion factors per unit concentrations of 4oK,
226Ra and 232Th in soils are 0.0395, 0.391 and 0.533 nGy h-I per Bq kg",
respectively. The overall mean activity concentrations of 4oK, 226Ra and 232Th
in the geological materials are 705, 65, and 163 Bq kg", respectively. The
estimated annual effective dose due to external exposure to terrestrial gamma
radiation varies from 0.06 to 2.00 mSv v'. with an avera'ge of 0.76 mSv fl.
Effective doses due to external exposures to cosmic radiation at ground levels
are calculated using modified analytical relations of Bouvilles and Lowders.
The values obtained range from 0.20 mSv i1 at sea level to 0.71 mSv v' at 2.5
km above sea level. A national average dose of 0.41 mSv i1 is also estimated
by considering the relief and the population distribution in Kenya.
Activity concentrations of222Rn in various sources of water: municipal and city
council water supplies, wells, springs, rivers and streams are measured using
conventional Liquid Scintillation Analysers. The mean and maximum activity
concentrations are 37 and 410 kBq m", respectively. These are much higher
than the world average of 10 kBq m-3. A screening survey also reveals that the
mean and maximum concentrations of 222Rnin indoor air are 100 and 1160 Bq
rn", respectively .. These are comparable to the internationally recommended
control levels (200-600 Bq m") and indicate existence of radon problem in
some dwellings. The annual effective doses due to internal exposures to
radiation varies from 0.36 to 5.90 mSv il for 222Rninhaled with air assuming
an equilibrium factor of 0.4, and from 0.01 to 0.75 mSv for 222Rningested with
drinking water also assuming a water consumption rate of 0.5 litre per day.
When the human population distribution is considered in -relation to the
people's living habits, and to the relief and the geological formations in the
country, it is concluded that the average annual effective dose from all natural
sources in Kenya is higher than the global average. In addition, ingestion of a
variety of soft rocks and other earth materials by some people, particularly
expectant women, is identified as another (unusual) internal exposure pathway . | en |
