Survey reports, equipment manuals, radiation-safety slide decks — the units shift from line to line: μGy, mSv, μSv/h. It looks like three separate systems. It is not. There are two units, plus one conversion. Once you know which question each of them answers, the table stops being intimidating.

01Two units, two different jobs

Gy (gray): how much energy was absorbed

This is pure physics. One kilogram of tissue absorbing one joule of radiation energy is one gray. It is what an instrument actually measures — regardless of what kind of radiation it is, who it landed on, or which organ it passed through.

Sv (sievert): how much harm it does to a person

The same amount of energy does different amounts of damage depending on the type of radiation and the organ it reaches. The sievert takes the gray figure and multiplies it by a weighting for the type of radiation and a weighting for the sensitivity of the tissue.

So the fundamental difference is this: the gray is measured, the sievert is calculated. Gy states a physical fact; Sv answers the question “what does that mean for a person?”

02One conversion: steps of a thousand

1 Sv = 1,000 mSv (millisievert) = 1,000,000 μSv (microsievert). The sievert itself is far too large to meet in daily work; what you actually see is the milli and micro levels.

03The ones with a slash are dose rates

There is a third style on the report: μGy/h, μSv/h. A slash means a dose rate — “how much per hour”. It describes how strong a particular spot is, not how much any particular person received.

The relationship is the one between speed and distance. Dose rate is the speed; dose is the distance covered. Standing in the same spot for an hour and for ten minutes are not remotely the same thing — a dose rate only becomes a dose once you multiply it by the time you spend there.

This is the unit an inspection body uses when it measures leakage radiation outside the X-ray room. They walk the instrument along the door seam, the viewing window and the walls, and the reading in μSv/h answers one question: how much would somebody standing here for an hour receive?

The same distinction sorts out the two instruments. Issue 7 covered them: a personal dosimeter monitors a person — you wear it, and at the end of a quarter it tells you your total, in mSv. A survey meter monitors a place — you hold it and read it on the spot, in μSv/h. One gives a dose, the other gives a dose rate. Similar names, similar looks, completely different jobs.

04Back to that survey report

With all of the above, reading the report comes down to three moves:

① Look for a slash after the unit

With a slash it is a dose rate (how strong a location is); without one it is a dose (a total accumulated over time). This step alone tells you what the number is talking about.

② Then read the prefix, m or μ

A factor of a thousand, and the single most common misreading. Get it right before you judge whether the figure is large or small.

③ Do not bother converting Gy to Sv

Under X-ray conditions the two numbers are equivalent, so when the report says μGy/h on one line and μSv/h on the next, you can read them as the same order of magnitude.

Once the units make sense, there is one more useful step: translating the numbers into something people can feel. That is what issue 2 does — expressing the dose of a single radiograph as “so many days of natural background” for a worried owner. Units are the ruler; the comparison is what you actually say out loud.

Once the table makes sense, the next annual inspection stops being a black box: you know what the instrument in the inspector’s hand is measuring, what the reading means, and what their conclusion is based on.

Source：Radiation weighting factors (1 for X- and gamma rays) and tissue weighting factors are given in ICRP Publication 103 (2007); the worldwide average natural background is about 2.4 mSv per year (UNSCEAR); an adult chest radiograph is roughly 0.1 mSv (RadiologyInfo.org, ACR/RSNA). Actual doses vary with equipment, technique and the individual, so the figures here are orders of magnitude for reference only and are not a compliance judgement.