Short answer. The gray (Gy) measures how much radiation energy body tissue absorbs; the sievert (Sv) measures the health risk of a dose, taking the type of radiation into account 13. One sievert is 1,000 millisieverts (mSv) 5. People worldwide receive about 2.4 mSv a year from natural sources 6. The WHO puts the threshold for acute radiation sickness at about 1 Sv (1,000 mSv) 1.
What is the difference between a gray and a sievert?
The two units answer different questions.
- Gray (Gy): how much energy was absorbed. The CDC defines the gray as a unit of absorbed dose, "an amount of energy deposited into a mass of tissue" 2.
- Sievert (Sv): how harmful that dose is likely to be. The WHO describes the sievert as the unit of effective dose "that takes into account the type of radiation" 1. The U.S. Department of Energy adds that effective dose also considers how sensitive the exposed tissue types are to damage 5.
To get from one to the other, scientists give each type of ionizing radiation a number called the quality factor (Q), based on how readily it transfers energy to the body's cells 3. In U.S. units, risk in rem equals the absorbed dose in rad multiplied by Q 3.
The CDC's clinical guidance on acute radiation sickness expresses doses in grays (and rads) 2. Agencies talking about everyday exposure, safety limits and long-term cancer risk work in sieverts and millisieverts 145.
How do sieverts, millisieverts, rem and rad convert?
The United States still uses older units alongside the international ones. The CDC says the SI units have replaced (or, for the curie, are replacing) the older ones, with fixed conversions 3:
| Quantity | International (SI) unit | Older U.S. unit | Conversion |
|---|---|---|---|
| Absorbed dose (energy in tissue) | gray (Gy) | rad | 1 Gy = 100 rad 23 |
| Effective dose (health risk) | sievert (Sv) | rem | 1 Sv = 100 rem 35 |
| Smaller steps | 1 Sv = 1,000 mSv | 1 rem = 1,000 mrem | 5 |
| Activity of a source | becquerel (Bq) | curie (Ci) | 1 Ci = 37 billion Bq 3 |
The becquerel and curie describe how much radiation a material gives off, not what a person receives 3. A reading in becquerels on a food test is a different kind of number from a dose in millisieverts.
How much radiation do people normally receive in a year?
Everyone receives a background dose from cosmic rays, the ground, radon in the air, and natural radioactivity in food and in the body 56. The figure depends on where you live and on medical exams.
| Reference figure | Annual dose |
|---|---|
| Worldwide average, natural sources | 2.4 mSv 6 |
| Japan average, natural sources | 2.1 mSv 6 |
| Canada average, natural background | 1.8 mSv 4 |
| United States, natural sources | about 3.1 mSv 5 |
| United States, all sources (natural plus medical and other) | about 6.2 mSv 5 |
| Japan, average medical exposure alone | 2.6 mSv 6 |
The WHO notes that exposure in some areas can be "more than 200 times higher than the global average" 1, and that medical use accounts for 98% of the population dose from human-made sources 1. In the U.S., radon gas alone contributes about 2.28 mSv a year on average 5.
What does a single exam or flight add?
Official examples give a sense of scale:
- A typical chest X-ray: 0.1 mSv 45.
- A typical cross-Canada flight: 0.02 mSv 4. A 3-hour jet flight: 0.01 mSv 5.
- A typical chest CT scan: 7 mSv 4. A cardiac CT scan: 20 mSv 5.
At what dose do acute effects begin?
Acute effects appear only after a large dose received over a short time. Agencies give slightly different round numbers:
- WHO: the threshold for acute radiation syndrome is "about 1 Sv (1000 mSv)" 1.
- CNSC (Canada): a dose that may cause symptoms of radiation sickness is "around 1000 mSv" 4.
- CDC (United States): acute radiation syndrome requires a dose greater than 0.7 Gy (70 rad), and mild symptoms may be seen from as low as 0.3 Gy (30 rad) 2.
The CDC also lists conditions beyond the number itself: the radiation must be penetrating (able to reach the internal organs), the entire body or a significant portion of it must receive the dose, and the dose must be delivered in a short time, usually a matter of minutes. Fractionated doses, delivered in small amounts over time, are less effective at causing the syndrome than a single dose of the same size 2.
The CDC gives these ranges for the full syndromes 2:
| Absorbed dose | Syndrome the CDC associates with it |
|---|---|
| 0.7 to 10 Gy | Bone marrow (blood-forming) syndrome 2 |
| 2.5 to 5 Gy | Dose estimated to kill half of those exposed within 60 days (LD50/60), bone marrow syndrome 2 |
| Above about 10 Gy | Gastrointestinal syndrome 2 |
| Above about 50 Gy | Cardiovascular and central nervous system syndrome 2 |
Below acute levels, the concern is long-term cancer risk. The WHO reports a significant increase in cancer risk above 100 mSv, and notes that some studies of children who had CT scans suggest the risk may increase even between 50 and 100 mSv 1. For safety rules, the U.S. Department of Energy explains that regulators assume conservatively that risk rises with dose from zero, with no threshold 5. Pregnancy matters: the WHO cites possible brain damage to the fetus from an acute dose above 100 mSv between weeks 8 and 15, or above 200 mSv between weeks 16 and 25 1.
How do dosimeters and survey meters read?
Two families of instruments answer two different questions:
- Survey meters measure the ambient dose rate at a place, or check body surfaces for contamination 7. The WHO says dose rate can be described in microsieverts per hour (µSv/h) or millisieverts per year (mSv/year) 1.
- Personal dosimeters give a cumulative reading: the total dose received while worn 7.
A rate tells you how fast dose is building up where you stand. A cumulative total tells you how much you have received so far. Check which of the two a display shows before comparing it with any figure in this guide.
Japan's Ministry of the Environment notes that results vary with the radiation level at the spot and with how the measurement is taken, and that converting readings into becquerels requires calibration at a facility with a reference radioactive source 7.
What official agencies recommend
Regulators set dose limits well below the levels linked to acute effects 145. These limits protect workers and the public; medical exposure is instead based on medical necessity as determined by the physician and patient 5.
| Agency | Public limit | Worker limit |
|---|---|---|
| CNSC, Canada | 1 mSv per year 4 | 50 mSv in any one year and 100 mSv over five consecutive years 4 |
| U.S. limits as summarized by the Department of Energy | 1 mSv (100 mrem) per year for the public and minors 5 | 50 mSv (5,000 mrem) per year 5 |
Where agencies differ, the gap is mostly rounding and scope. The WHO and CNSC put acute sickness near 1,000 mSv 14, while the CDC gives a clinical threshold of 0.7 Gy with mild symptoms possible from 0.3 Gy 2. Background figures also differ by country: 1.8 mSv in Canada 4, 2.1 mSv in Japan 6, and about 3.1 mSv from natural sources in the U.S. 5, against a worldwide average of 2.4 mSv 6.
In an emergency, the CDC advises getting inside, staying tuned for updated instructions from emergency response officials, and not leaving shelter until officials or responders say it is safe 8.
Frequently asked questions
Is a gray the same as a sievert?
No. The gray measures energy absorbed by tissue 2; the sievert measures health risk and depends on the type of radiation 1. They are linked by a quality factor assigned to each radiation type 3.
Is 1 mSv dangerous?
One millisievert is the annual public limit in Canada and the U.S. 45, and less than half the worldwide average natural dose of 2.4 mSv 6. The WHO links a significant increase in cancer risk to doses above 100 mSv 1.
How many millisieverts cause radiation sickness?
The WHO and CNSC both put the threshold at about 1,000 mSv 14. The CDC uses 0.7 Gy received over a short time, with mild symptoms possible from 0.3 Gy 2.
What does µSv/h on a meter mean?
It is a dose rate: microsieverts per hour at that spot 1. A personal dosimeter shows a cumulative total instead 7.
How much radiation is a CT scan compared with a year of background?
A typical chest CT is about 7 mSv 4, roughly three years of the 2.4 mSv worldwide natural average 6. A cardiac CT is about 20 mSv 5.
Get a plan for your household
Knowing the units helps you understand official alerts. The Phaenyx personal plan gathers your country's official guidance on radiation emergencies, sheltering and evacuation into one document for your household. Build your Phaenyx plan.
Sources
- World Health Organization (WHO) — Ionizing radiation and health effects — https://www.who.int/news-room/fact-sheets/detail/ionizing-radiation-and-health-effects (accessed 2026-10-20)
- U.S. Centers for Disease Control and Prevention (CDC) — Acute Radiation Syndrome: Information for Clinicians — https://www.cdc.gov/radiation-emergencies/hcp/clinical-guidance/ars.html (accessed 2026-10-20)
- U.S. Centers for Disease Control and Prevention (CDC) — Measuring Radiation — https://www.cdc.gov/radiation-emergencies/php/ph-professionals/measuring-radiation.html (accessed 2026-10-20)
- Canadian Nuclear Safety Commission (CNSC) — Radiation doses — https://www.cnsc-ccsn.gc.ca/eng/resources/radiation/radiation-doses/ (accessed 2026-10-20)
- U.S. Department of Energy, Office of Environment, Health, Safety & Security — Radiation in Perspective — https://www.energy.gov/documents/radiationinperspectivepdf (accessed 2026-10-20)
- Ministry of the Environment, Japan — Exposure Dose from Natural and Artificial Radiation — https://www.env.go.jp/en/chemi/rhm/basic-info/2024/02-05-01.html (accessed 2026-10-20)
- Ministry of the Environment, Japan — Instruments for Measuring External Exposure — https://www.env.go.jp/en/chemi/rhm/basic-info/2024/02-04-05.html (accessed 2026-10-20)
- U.S. Centers for Disease Control and Prevention (CDC) — Preparing for a Radiation Emergency — https://www.cdc.gov/radiation-emergencies/safety/index.html (accessed 2026-10-20)
Turn this into action
A plan worked out for your household.
- Water, food and equipment counted for each person and pet
- The nuclear plants and zones around your home
- Phaenyx Safe: one tap to tell your people you are safe
Sources
- World Health Organization (WHO) · Ionizing radiation and health effects checked 2026-10-20
- U.S. Centers for Disease Control and Prevention (CDC) · Acute Radiation Syndrome: Information for Clinicians checked 2026-10-20
- U.S. Centers for Disease Control and Prevention (CDC) · Measuring Radiation checked 2026-10-20
- Canadian Nuclear Safety Commission (CNSC) · Radiation doses checked 2026-10-20
- U.S. Department of Energy, Office of Environment, Health, Safety & Security · Radiation in Perspective checked 2026-10-20
- Ministry of the Environment, Japan · Exposure Dose from Natural and Artificial Radiation checked 2026-10-20
- Ministry of the Environment, Japan · Instruments for Measuring External Exposure checked 2026-10-20
- U.S. Centers for Disease Control and Prevention (CDC) · Preparing for a Radiation Emergency checked 2026-10-20