Notes on Radiation Dosage, Dosimetry, and the Radon Problem

INTRODUCTION:

The biological effects of radiation arise from the absorption of the radiant energy to produce heat, electronic excitation and/or ionization.

Radio, T.V., microwave, visible light, u.v. light, x-rays, $\gamma$-rays are all electromagnetic radiation and differ only in wavelength. Electromagnetic radiation may have both beneficial and harmful effects: e.g. u.v. light absorbed by the skin can supply needed vitamin D, but excess u.v. radiation accounts for much skin cancer. X-rays and $\gamma$-rays are more penetrating and so can affect tissue below the skin.

Besides electromagnetic radiation one has high velocity charged (and neutral) particles. From naturally radioactive materials, the charged particles are either high speed electrons ($\beta$'s, beta rays) or alpha particles ($\alpha$'s, the nuclei of helium atoms). Both types are rather easily stopped by a small thickness of matter e.g. $\sim$ 1.8 mm Al stops 1.17 MeV $\beta$'s from RaE, and $\sim$ 0.06 mm Al stops 5.3 Mev $\alpha$'s from Po. Hence natural radioactivities are normally of little concern unless the parent nucleus has been inhaled or ingested in the body. The biological effects of neutral particles (e.g. neutrons and neutrinos) from naturally radioactive materials are normally negligible.

Radioactivity unit:

1 becquerel (Bq) = 1 disintegration/s $\cong$ 27 pCi (picoCurie)

Radiation dose

is the radiant energy absorbed per unit mass.

    Dose UNITS:

0.01 J of radiation absorbed/kg of mass = 1 rad
1 J of radiation absorbed/kg of mass = 1 gray (abbreviated Gy)

    Dose Equivalent:

includes the long term relative biological effects of different types of radiation. The original unit was the rem (rad equivalent man), but the now recommended S.I. unit is the sievert (abbreviated Sv) with:
1 Sv = 100 rem = 10$^5$ mrem
and so: 1 mSv = 100 mrem

Federal laws on permissible doses are:

1. For workers, $<$ 50 mSv/year for a whole body dose, but employer must follow the ALARA (As Low As Resonably Achievable) principle. For the hands alone, 750 mSv/yr are allowed.
2. For the general population $<$ 5 mSv/yr whole body dose.

RADIATION SOURCES:

Besides the sun's u.v. radiation, the natural environment contributes an unavoidable dose equivalent to $\sim$ 1.3 mSv/yr plus a variable dose from inhaled radon which often is several times larger: see The Radon Problem (below). Hence the average natural background radiation dose is $\sim$ 3 mSv/yr. About .25 mSv/yr of this dose comes from internal radioactivities in the body (chiefly $^{40}$K which constitutes 0.0119% of natural K and has T $_{1/2} \sim 10^{9}$ years). The rest comes from external natural radioactivities in the earth (chiefly decay chains of uranium and thorium, T $_{1/2} \sim 4 \times 10^{9}$ years and T $_{1/2} \sim 10^{10}$ years) and from cosmic rays. The cosmic ray contribution increases with altitude and is $\sim$ 30 mSv/yr at 40,000 feet elevation (jet airplane altitudes).

Brick and stone houses often have larger backgrounds. Living in Denver (elevation 5200 ft.) contributes an additional $\sim$ 0.7 mSv/yr. In the Kerala region of India and the Espirito Santo region of Brazil, natural sources give $\sim$ 30 mSv/yr with no obvious abnormality resulting to the indigenous population so the Federal regulations seem very conservative for the general population.

Man-made radiation exposure averages $\sim$ .7 mSv/yr and comes almost exclusively from medical and dental x-rays. A single dental x-ray may involve 7 mSv to the skin. Exposures from nuclear power generating stations are nearly zero. In fact, per KWH of electricity generated, the radioactivity released from coal fired plants is often high compared to that permitted from nuclear plants since many coals contain appreciable uranium and/or thorium plus the equilibrium decay products from these long-lived radioactive nuclei.

For perspective on radiation exposure, Prof. Cameron of the UW Medical Physics Dept. suggests translating doses into a natural unit, the BERT defined as the Background Equivalent Radiation Time. Thus a BERT equal to 1 yr would correspond to 3 mSv (see above discussion on average background dose).

THE RADON PROBLEM:

Radon (an inert gas) from decay of naturally occurring U and Th in the earth continually diffuses into the atmosphere and may cause $\sim$ 10,000 lung cancer cases per year in the U.S. The radon content of outdoor air 1 meter above ground typically gives 4 to 15 becquerels/m$^3$ The health effects come mainly from inhalation of $^{222}$Rn (from U) since this radon isotope has a T$_{1/2}$ of 3.82 days whereas the thorium radon isotope ($^{220}$Rn) has T$_{1/2}$ of only 56 seconds. The indoor air concentration of radon ($\sim$ 50 Bq/m$^3$) varies perhaps a factor of a thousand from location to location, depending upon the U content and physical characteristics of the soil, moisture content, building construction, winds, etc. (A house in Maine had a record $\sim$ 160,000 Bq/m$^3$!)

A radon concentration of 50 Bq/m$^3$ may result in an annual dose equivalent to bronchial epithelium (site of most radiation induced lung cancer) of $\sim$ 2.5 mSv/yr. Perhaps 25% of Wisconsin houses have concentrations $>$ 150 Bq/m$^3$ which is the EPA guideline where action should be taken in a few years since the lung cancer risk may be comparable to smoking 3 to 10 cigarettes/day. (M.S. Blumenthal, Wisconsin Medical Journal, Vol. 87, May 1988, p.17) In fact 2% of U.S. homes have radon concentrations $>$ 300 Bq/m$^3$ and occupants should take action to reduce the concentration since they may be receiving an effective dose of $> \sim$ 16 mSv/yr. (By contrast the EPA limit for off-site exposure from nuclear reactors or from nuclear waste depositories is only .25 mSv/yr!) (Bodansky, Physics and Society 16, No. 4, p. 6, 1987).

If a house has high radon levels, then the radon ingress usually is from air infiltration from soil beneath the house. Natural convection in the house (chimney effect) tends to pull in the radon contaminated air. A solution is to drive pipes through the basement floor and connect them to fans exhausting to the outside air. See A. V. Nero Jr. ``The Indoor Radon Story'', Technology Review Vol.89, No. 1, p. 28, (1986); also A. Nero, ``Earth, Air, Radon and Home'', Physics Today, Vol 42, No. 4, p.32, (1989).

A good general reference on the subject is ``Radon and its Decay Products in Indoor Air'' edited by William Nazaroff and A.V. Nero, Jr., John Wiley & Sons, 1988. Ground water supplies often contain high concentrations of radon from uranium decay in the aquifers. The radon concentration in public ground water supplies averages $\sim$ 5000 Bq/m$^3$, and is much higher in some of the New England states. The health hazard is apparently not from drinking the water, but from the water's contribution to the indoor radon air problem: perhaps $\sim$ 5 Bq/m$^3$. Private wells often have high concentrations. Storage or aeration of the water provides effective control of the hazard.

OTHER ISSUES:

Is a Small Amount of Radiation Healthy - The Hormesis Effect

The following is from RADIATION DOSIMETRY by Prof. John R. Cameron, Department of Medical Physics, UW, Madison (1989):

``Studies on nuclear workers often show that they have less cancer than other member of the population and even of other workers with similar jobs. This is usually explained as the 'healthy worker' effect. That is, for reasons not understood, radiation work attract healthy workers. An alternate explanation which is rarely mentioned is the possibility that a small amount of radiation is good for you. This is referred to as the 'hormesis' effect. Since humans and all of our ancestors evolved in a sea of natural radiation, it is possible that mutations have occurred that produce the hormesis effect. Animal experiments have demonstrated the hormesis effect. Rats exposed to increased radiation have a longer survival than their controls.''