Gray (Gy) - measures energy deposited per kilogram of tissue. Think of it like measuring how many bullets hit a target, not how much damage they do.

Sievert (Sv) - measures biological damage. This accounts for the fact that different types of radiation and different tissues react differently. Think of this as the actual damage done.

The bullet's energy is identical in all cases (same Gy), but the biological damage varies wildly (different Sv).

The same energy deposited (Gy) causes vastly different biological damage (Sv) depending on:

What tissue (bone marrow is like your heart - critical; muscle is more resilient)

What radiation type (alpha particles are like hollow-point bullets - more damaging per energy unit; gamma rays are like full metal jacket - cleaner pass-through)

For most medical purposes (X-rays, gamma rays), 1 Gy is approx 1 Sv, which is why people use them interchangeably and add to confusion.

Location and delivery matter enormously. It's like pouring water. Put 3 liters in your lungs, you drown (dead). Put 3 liters on your hand, your hand gets wet (annoying but harmless).

3 Gy to your whole body at once is potentially fatal. You'll likely die within weeks from bone marrow failure, your blood cells can't regenerate. 3 Gy to a small tumor in your knee is a typical treatment session. The rest of your body gets almost nothing, and your bone marrow keeps working fine. 3 Gy spread over 6 sessions (0.5 Gy each) to a localized area is a very low dose that gives tissue time to repair.