CIH Radiation Units: Imperial vs. SI Conversion Guide
The radiation chapter of the CIH exam throws four different unit systems at you — Curie, Roentgen, rad, rem — each paired with an SI equivalent. They're confusing on their own, but they stop being confusing once you see them as a chain: each unit describes radiation at a different point in its journey from the source to a person.
Ionizing radiation units aren't four random terms to memorize — they're four checkpoints along one path: Activity (Curie/Becquerel) at the source → Exposure (Roentgen) in the air → Absorbed Dose (rad/Gray) in a material → Biological Dose (rem/Sievert) in a person. Imperial and SI units exist side by side because SI is the international standard, but imperial units (Curie, rad, rem) are still common in industrial and historical contexts — and the conversions between them are almost always clean factors of 100.
The Journey of Ionizing Radiation and Its Units
Before memorizing conversion factors, it helps to see the whole path radiation takes — from the radioactive source, through the air, into tissue, and finally to the biological effect on a person. Each of the four unit categories below sits at one stage of that journey.
Ionizing Radiation Units and Relationships
| Category | Imperial Unit | SI Unit | Relationship / Conversion | Description & Purpose | Measurement Type |
|---|---|---|---|---|---|
| Activity | Curie (Ci) | Becquerel (Bq) | 1 Ci = 3.7 × 1010 Bq | Measures the number of atoms disintegrating per unit of time at the source. | Direct Measurement |
| Exposure | Roentgen (R) | N/A | 1 R = 2.58 × 10-4 C/kg air | Measures ionization produced in dry air by γ or X-ray radiation. Does not measure dose in the body. | Direct Measurement |
| Absorbed Dose | rad | Gray (Gy) | 1 Gy = 100 rad | Measures the amount of radiation absorbed by a mass (1 rad = 100 erg/g), regardless of radiation type. | Direct Measurement |
| Biological Dose | rem | Sievert (Sv) | 1 Sv = 100 rem | Measures the dose equivalent, or biological impact — factors in radiation type using a Quality Factor (QF). | Calculated |
Explanation of Conversions and "Why"
The units for ionizing radiation follow a logical progression based on the journey radiation takes — starting at its radioactive source, moving through the air, being absorbed by a target, and finally causing a biological effect.
1. Direct Measurements
These units represent physical phenomena that can be directly observed and quantified using specific detection instruments.
At the Source: Activity Direct Measurement
The journey begins at the radioactive material itself. Activity measures how many atoms are disintegrating (decaying) every second within that source.
Imperial Unit: Curie (Ci), defined as 3.7 × 1010 disintegrations per second.
SI Unit: Becquerel (Bq), which is simply 1 disintegration per second.
How it's measured: instruments like a gas proportional counter or a Geiger-Müller (GM) meter detect these disintegration counts directly.
In the Air: Exposure Direct Measurement
Once radiation is emitted, it travels through the environment. For gamma and X-ray radiation, we measure the amount of ionization it produces in the surrounding dry air.
Unit: Roentgen (R). This represents the amount of charge produced in 1 cm³ of dry air.
How it's measured: typically with an ion chamber, which captures the voltage difference created by the positive and negative ions produced.
Key Note: This measures exposure only in the air and not in the body, and there is no direct SI equivalent to the Roentgen.
Striking the Target: Absorbed Dose Direct Measurement
When radiation hits a physical mass, such as human tissue, it is absorbed. The Dose measures the actual amount of energy deposited into that mass.
Imperial Unit: rad (radiation absorbed dose), where 1 rad equals 100 erg/g of mass.
SI Unit: Gray (Gy), where 1 Gray is equal to 100 rad.
How it's measured: devices like Thermoluminescent Dosimeters (TLDs) or calibrated detectors measure this energy deposition directly — for example, by measuring the light emitted when a TLD crystal is heated.
Limitation: this unit measures pure energy absorption but does not differentiate between the types of radiation or their specific biological impacts.
2. Calculated Units
These units are derived by applying a mathematical factor to a direct measurement, to account for biological variables.
The Human Impact: Biological Dose Calculated
The final stage of the progression is determining the actual biological damage to a person. This is known as the dose equivalent, and unlike the first three categories, it isn't read directly off an instrument — it's calculated based on the Absorbed Dose, multiplied by the Quality Factor. Because different types of radiation (like alpha vs. gamma) cause different amounts of damage to human tissue even when the absorbed energy is the same, a Quality Factor (QF) must be applied.
Imperial Unit: rem (radiation equivalent). Calculation formula: rem = Number of radsAbsorbed Dose × QF.
SI Unit: Sievert (Sv), where 1 Sv equals 100 rem — calculated from Grays using the same biological weighting factors.
The "Why": if you have a direct measurement in rads, you multiply it by the QF (e.g., 1 for X-rays, 10 for high-speed neutrons) to determine the rem, the actual biological impact. X-rays have a QF of 1 while high-speed neutrons have a QF of 10 — meaning neutrons cause ten times the biological damage for the same absorbed dose.
Following along with my real CIH prep?
This page came out of my own Ch.5 Radiation notes — I found the unit system genuinely confusing until I mapped it as a chain instead of four separate facts. Full context, plus the rest of the radiation chapter breakdown, is in the CIH Exam Journal.
Read the CIH Exam JournalWant to practice these radiation equations? The CIH Equation Master has all of them — free, no sign-up.
CIH Radiation Units — Frequently asked questions
What are the units of radiation on the CIH exam?
Four categories, each with an imperial and an SI unit: Activity (Curie / Becquerel), Exposure (Roentgen, no direct SI equivalent), Absorbed Dose (rad / Gray), and Biological Dose (rem / Sievert). Each stage describes radiation at a different point in its journey from the source to a person.
How do you convert rad to rem?
rem = rad × Quality Factor (QF). The QF weights the absorbed dose by how biologically damaging that type of radiation is: QF = 1 for X-rays, gamma, and beta; QF = 10 for high-speed neutrons; QF = 1–20 for alpha particles.
Why is the Roentgen unit different from the others?
The Roentgen only measures ionization produced in dry air — not dose absorbed by the body — which is why it has no direct SI equivalent. SI measures air exposure directly in coulombs per kilogram (C/kg) instead of defining a parallel unit.
What is 1 Curie in Becquerels?
1 Curie (Ci) = 3.7 × 1010 Becquerels (Bq). Both measure activity — the number of atoms disintegrating per second at the radioactive source.
