2.2 Gases and Hygiene
39
Radioactive Gases: Radon
Rn is a colourless, odourless and tasteless gas that is found naturally in rocks and
soils. Of the 37 known isotopes of radon,
195 Rn to
231 Rn, the commonest is
222 Rn.
Nowadays, this gas is considered as one of the most significant causes of lung cancer
(ICRP 2011; WHO 2009). The literature provides different reference values for its
effects on the health of miners (e.g. Navaranjan et al. 2016; Kreuzer et al. 2017).
A first occupational reference value can be obtained by measuring radioactivity
levels. An annual average equivalent to more than 300 radioactive decays per m
3
(300 Bq m
−3 ) indicates the need to restrict gas exposure (Shahrokhi et al. 2017).
Other references (e.g. DHHS 1987) suggest values above 0.11–0.25 WL.
5 In terms
of gas doses absorbed by living matter, the recommended parameters range from 1 to
20 mS year
−1 (ICRP 2011). Among the protective measures that have been considered
to reduce concentrations of this gas are (ICRP 2011): (a) adequate selection of the
exploitation method (such as mining by remote control); (b) intense ventilation; (c)
general air filtering;
6 (d) individual protection equipment for dust filtering; and (e)
work rotation to reduce exposure time.
2.2.4 Weight of Gases and Stratification
In mine-ventilation calculations, the weight of air is usually assumed to be 1.2 kg
m
−3 , based on 1 atm of pressure, a temperature of 15 °C and a 60% humidity.
The diffusion of gases consists of a net movement of gas molecules from zones
of higher gas concentration to zones of lower concentration. A gas’s speed of diffusion is inversely proportional to the square root of its molar mass (Graham’s law).
Gases’ diffusivity increases as temperature rises (Chapman–Enskog theory). The
effect of pressure on diffusivity is insignificant except at very high pressures (>20
atmospheres). Thus, the diffusivity is considered as a constant in the context of mine
ventilation.
Gases that undergo a diffusion or mixing process do not usually become stratified according to density. This is because of the predominance of random thermal
movements of molecules, which produce forces that are much more significant than
those of gravity. This phenomenon is very similar to that which can be observed in
the case of “fresh air” gases, which do not become separated due to density difference (Badino 2009). Gas-stratification occurs when gases do not have enough time
to inter-diffuse. Thus, if a gas leak has occurred only recently, the mixture may not
5 A WL (Working Level) equals 2.08 x 10 −5 J m −3 of potential alpha energy concentration of Rn
progeny. One WLM −1 (Working Level Month) equals to 1WL · 173 h or 0.5 WL ·340 h, as 173 h
of work a month are established.
6 Although it is difficult to eliminate Rn because of its gaseous nature, its decay products are solid and
usually adhere to dust particles. They can, therefore, be filtered and precipitated electrostatically.
39
Radioactive Gases: Radon
Rn is a colourless, odourless and tasteless gas that is found naturally in rocks and
soils. Of the 37 known isotopes of radon,
195 Rn to
231 Rn, the commonest is
222 Rn.
Nowadays, this gas is considered as one of the most significant causes of lung cancer
(ICRP 2011; WHO 2009). The literature provides different reference values for its
effects on the health of miners (e.g. Navaranjan et al. 2016; Kreuzer et al. 2017).
A first occupational reference value can be obtained by measuring radioactivity
levels. An annual average equivalent to more than 300 radioactive decays per m
3
(300 Bq m
−3 ) indicates the need to restrict gas exposure (Shahrokhi et al. 2017).
Other references (e.g. DHHS 1987) suggest values above 0.11–0.25 WL.
5 In terms
of gas doses absorbed by living matter, the recommended parameters range from 1 to
20 mS year
−1 (ICRP 2011). Among the protective measures that have been considered
to reduce concentrations of this gas are (ICRP 2011): (a) adequate selection of the
exploitation method (such as mining by remote control); (b) intense ventilation; (c)
general air filtering;
6 (d) individual protection equipment for dust filtering; and (e)
work rotation to reduce exposure time.
2.2.4 Weight of Gases and Stratification
In mine-ventilation calculations, the weight of air is usually assumed to be 1.2 kg
m
−3 , based on 1 atm of pressure, a temperature of 15 °C and a 60% humidity.
The diffusion of gases consists of a net movement of gas molecules from zones
of higher gas concentration to zones of lower concentration. A gas’s speed of diffusion is inversely proportional to the square root of its molar mass (Graham’s law).
Gases’ diffusivity increases as temperature rises (Chapman–Enskog theory). The
effect of pressure on diffusivity is insignificant except at very high pressures (>20
atmospheres). Thus, the diffusivity is considered as a constant in the context of mine
ventilation.
Gases that undergo a diffusion or mixing process do not usually become stratified according to density. This is because of the predominance of random thermal
movements of molecules, which produce forces that are much more significant than
those of gravity. This phenomenon is very similar to that which can be observed in
the case of “fresh air” gases, which do not become separated due to density difference (Badino 2009). Gas-stratification occurs when gases do not have enough time
to inter-diffuse. Thus, if a gas leak has occurred only recently, the mixture may not
5 A WL (Working Level) equals 2.08 x 10 −5 J m −3 of potential alpha energy concentration of Rn
progeny. One WLM −1 (Working Level Month) equals to 1WL · 173 h or 0.5 WL ·340 h, as 173 h
of work a month are established.
6 Although it is difficult to eliminate Rn because of its gaseous nature, its decay products are solid and
usually adhere to dust particles. They can, therefore, be filtered and precipitated electrostatically.