Unit 2SafetyObjective SAF-C11 min read

How Radiation Harms: Ionization, Dose Quantities and Units, Occupational and Public Limits

Unit 2: Radiation, Magnetic, and Facility Safety. Lesson 6 of 60 in CRES Academy.

Every protective feature you maintain exists because x-rays ionize tissue. This lesson is the vocabulary and the numbers behind all of it, and it is where the unit-conversion questions come from.

Ionization and what it damages

An x-ray photon with enough energy knocks an electron off an atom, leaving an ion pair. That is ionization, and it is the difference between x-rays and the radio waves a wireless access point puts out. X-rays and gamma rays are electromagnetic radiation; alpha particles, beta particles, and neutrons are particulate radiation, which carries mass, gives up its energy over a very short distance, and is what a nuclear medicine source emits inside the patient. Damage happens two ways: the direct effect is a photon hitting a critical molecule such as DNA, and the indirect effect is a photon splitting water into reactive fragments that then attack the molecule. Because the body is mostly water, the indirect effect does most of the harm.

Somatic damage is to the exposed person's own tissues; genetic damage is to the reproductive cells and so to descendants.
Dividing cells are the most sensitive, which is why bone marrow, the gonads, and children matter most.
Dose rate matters: the same total dose delivered slowly does less harm than the same dose all at once, because repair has time to work.

Two kinds of effect sit behind every limit in this course. Stochastic effects, such as cancer and genetic change, have no threshold: any dose carries some probability of them, and the probability rises with dose while the severity does not. Deterministic effects, such as skin reddening, hair loss, and cataract, need a threshold dose before they appear at all, and above it they get worse as dose rises. Occupational and public limits exist to keep stochastic risk small; the dose alerts on a long fluoroscopic case exist to keep deterministic injury from happening at all.

Quantities and units

Exposure, air kerma   roentgen (R); 1 R = 2.58 x 10 to the minus 4 C/kg; air kerma in gray is the SI form
Absorbed dose         gray (Gy) or rad;      1 Gy = 100 rad
Equivalent dose       sievert (Sv) or rem;   1 Sv = 100 rem
Effective dose        sievert, weighted for radiation type and tissue sensitivity
Prefixes              milli is a thousandth, micro a millionth; 1 mSv = 1000 microsieverts

Work the conversions both ways. 5 rem is 50 mSv. 0.2 mSv is 20 mrem. 2 Gy is 200 rad.

Where background dose comes from

Natural background is about 3 mSv per year, radon being the largest part, and the largest artificial source for the population is medical imaging. One chest image is about 0.02 mSv, so a staff member's annual dose is best understood against background rather than against zero.

The limits

Occupational effective dose     50 mSv per year, cumulative 10 mSv x age in years
Lens of the eye                 150 mSv per year
Thyroid, skin, hands, and feet  500 mSv per year
Public                          1 mSv per year frequent, 5 mSv infrequent
Embryo or fetus                 5 mSv over the gestation, 0.5 mSv in any month once declared
Education and training under 18 1 mSv per year

A worked example: a badge reporting 0.4 mSv in a quarter projects to about 1.6 mSv a year, which is a few percent of the occupational limit and is a normal result for a radiographer; the same reading on a badge that never leaves the booth says something is wrong with the badge or the room.

Personnel monitoring

Badges are thermoluminescent or optically stimulated detectors read by a service, usually quarterly, and worn at the collar outside the apron so they read what reaches the unprotected neck and head. A thermoluminescent badge stores the energy it absorbs in a crystal and gives it back as light when the reader heats it, which is thermoluminescence; the properties of the crystal are the reason it is used, because lithium fluoride absorbs radiation much the way tissue does. In pregnancy a second badge goes at the waist under the apron. The reports go to the radiation safety officer and the committee, who look for trends, for a badge left in the room, and for anyone approaching an investigation level. If you move a badge rack or leave your own badge on a cabinet, you have corrupted the record.

Exam tip: 1 Gy = 100 rad and 1 Sv = 100 rem, so divide rem by 100 to get sieverts. Occupational 50 mSv a year, cumulative 10 mSv times age, lens 150, extremities and skin 500, public 1, declared pregnancy 0.5 mSv a month. Background is about 3 mSv a year.

Remember

X-rays ionize, the indirect effect through water does most of the damage, somatic harm is to you and genetic harm to descendants, and dividing cells are the most sensitive; 1 Gy is 100 rad and 1 Sv is 100 rem; background runs about 3 mSv a year; and the limits are occupational 50 mSv a year with a cumulative 10 mSv times age, lens 150 mSv, skin and extremities 500 mSv, public 1 mSv frequent and 5 mSv infrequent, a declared embryo or fetus 0.5 mSv a month, and trainees under 18 at 1 mSv.

Deeper explanation

How Radiation Harms: Ionization, Dose Quantities and Units, Occupational and Public Limits, explained slowly

Ionization is the whole reason the rules exist

A radio transmitter and an x-ray tube both put out electromagnetic energy. The difference is that a single x-ray photon carries enough energy to strip an electron from an atom, leaving an ion pair and a chemically reactive mess behind. That is ionization, and it is why there are dose limits for one and not the other.

The direct effect is a photon striking a critical molecule, above all DNA.
The indirect effect is a photon splitting water into reactive fragments that then attack the molecule. Because the body is mostly water, this route does most of the damage.
Somatic damage is to the exposed person. Genetic damage is to reproductive cells, and so to descendants.
Dividing cells are most sensitive, which ranks bone marrow, the gonads, and children at the top of the risk list.
Dose rate matters: the same total dose given slowly does less harm than the same dose at once, because repair mechanisms have time to work. That is the biological reason pulsed fluoroscopy is more than a dose arithmetic trick.

Two kinds of effect, and which limit answers which

Every number in the limit table exists to hold down one of two very different risks, and the exam expects the two words.

Stochastic effects     cancer and genetic change; no threshold, so any dose carries some
                       probability; more dose raises the chance, not the severity
Deterministic effects  skin reddening, hair loss, cataract, reduced fertility; a threshold
                       must be passed first, and above it severity rises with dose

Occupational and public limits are written against stochastic risk, which is why they are low, annual, and cumulative. The alerts and trigger levels on a long interventional case are written against deterministic injury, which is why they are about one patch of skin during one procedure rather than a yearly total. A technician who keeps those apart can answer the common exam trap: a dose limit protects a worker over a career, while a skin dose trigger protects one patient during one case.

The quantities, in the order a question walks them

Quantity          Asks                                  Units and conversions
Exposure,         how much ionization in air            roentgen (R); 1 R = 2.58 x 10 to the
air kerma                                               minus 4 C/kg; air kerma in gray is the SI form
Absorbed dose     how much energy per kg of tissue      gray (Gy), or rad; 1 Gy = 100 rad
Equivalent dose   absorbed dose weighted for how        sievert (Sv), or rem; 1 Sv = 100 rem
                  damaging that radiation type is
Effective dose    equivalent dose weighted for which    sievert
                  tissues were hit
Prefixes          milli a thousandth, micro a           1 mSv = 1000 microsieverts = 100 mrem
                  millionth

Practice the conversions in both directions until they are reflexes: 5 rem is 50 mSv; 50 mSv is 5 rem; 0.2 mSv is 20 mrem; 2 Gy is 200 rad; 300 mrem is 3 mSv. In diagnostic imaging the weighting factor for x-rays is one, so an absorbed dose in milligray and an equivalent dose in millisievert come out numerically the same, which is why the two units get used loosely in conversation and precisely on exams.

Background, and the scale of the numbers

Natural background is about 3 mSv per year, with radon the largest single contributor, and the largest artificial contribution to population dose is medical imaging. One chest image is about 0.02 mSv. Those two numbers together are the only honest way to explain a staff dose report: a badge reading a fraction of a millisievert in a quarter is a small addition to what the person received anyway from the ground they stand on.

The limits

Who or what                        Limit
Occupational, effective dose       50 mSv per year
Occupational, cumulative           10 mSv x age in years
Lens of the eye                    150 mSv per year
Thyroid, skin, hands, and feet     500 mSv per year
Public                             1 mSv per year frequent exposure, 5 mSv infrequent
Embryo or fetus                    5 mSv over the gestation; 0.5 mSv in any month once declared
Education and training, under 18   1 mSv per year

Worked example on the cumulative limit: a 40-year-old technologist's lifetime occupational limit is 10 x 40 = 400 mSv. Worked example on a badge: a quarterly report of 0.4 mSv projects to about 1.6 mSv a year, roughly 3 percent of the annual limit and a normal figure for someone doing fluoroscopy properly. The same badge reading 15 mSv in a quarter projects to 60 mSv, over the limit, and is an investigation: was the badge left in a room, worn under the apron, or is somebody standing in the wrong place during runs.

Personnel monitoring, as a process

Badges are TLD or OSL detectors, read by an outside service, normally on a quarterly cycle. They are worn at the collar outside the apron, because that reads what reaches the unprotected neck, eyes, and thyroid, which is where the limits that bite are. A declared pregnant worker wears a second badge at the waist under the apron. Reports go to the RSO and the radiation safety committee, who look for trends, outliers, and the signature of a badge that never went home. For you there are two rules: your badge stays on you and never on a cabinet in the room, and if you move a badge rack during a renovation, tell the officer where it went.

What to memorize

Ionization, direct and indirect effect, somatic and genetic, dividing cells most sensitive, dose rate matters.
1 Gy = 100 rad, 1 Sv = 100 rem, 1 mSv = 1000 microsieverts.
Occupational 50 mSv per year and 10 mSv times age cumulative; lens 150; thyroid, skin, hands, feet 500; public 1 and 5; declared pregnancy 0.5 per month; under 18, 1 mSv.
Background about 3 mSv a year; one chest image about 0.02 mSv.
Badge at the collar outside the apron, read quarterly, reviewed by the officer and the committee.