Page 137 - Environmental Control in Petroleum Engineering

124 Environmental Control in Petroleum Engineering

through a body. Thus, the dose from gamma rays is a whole body
dose, and all organs can be exposed. The dose from neutrons is very
complex and will not be discussed.
There are two major types of biological effects of radiation: those
affecting cells as a whole and those affecting the reproductive capacity
of the cells.
The major effect on cells as a whole is for the radiation to break
chemical bonds within the cell and create free radicals. The most
common free radicals are those created from the decomposition of
water, hydrogen and hydroxyl:

+
H 2O -> H + OH

Hydroxyls can combine to form hydrogen peroxide:

OH + OH- -» H 2O 2

Hydrogen peroxide is highly reactive and can react with most other
molecules in the cell, disrupting the cellular chemistry.
The most important genetic changes involve cellular reproduction.
If radiation breaks or alters the DNA molecules within a cell, the
ability of the cell to replicate itself is impaired. In most cases, any
alteration in the DNA prevents it from reproducing. In some cases,
however, the cell is able to reproduce, but the subsequent cells may
be mutated. Similar mutations in cells also arise from the elevated
temperatures of cooking, from drugs, and from exposure to chemicals.
Experience has shown that a one-time whole body dose of less than
about 50 REM will not result in any noticeable or measurable acute
effects. A dose an order of magnitude higher, e.g., 400-500 REM, is
a lethal dose for 50% of those receiving it. Thus, the LD 50 for humans
is about 500 REM, with death usually occurring about two months
after exposure. A dose above 1,000 REM is considered lethal for all
exposed, i.e., LD 100 is about 1,000 REM.
Long-term effects from a single large exposure or from a chronic
low-level exposure may include loss of hair, eye cataracts, cancer, or
leukemia. Unfortunately, those effects also arise from causes other than
nuclear radiation so it is difficult to determine whether or not they
come from radiation exposure. In many cases, no effects will be
observed following exposure to radiation.