Blood count can be depressed with a whole-body dose of:

• A. 1 Sv
• B. 0.25 Sv
• C. 25 Sv
• D. 10 Sv

Answer: (B)

The blood-forming system is highly sensitive to ionizing radiation, so bone marrow damage from whole-body exposure leads to a drop in circulating blood cells. At about a quarter of a sievert, the bone marrow’s production of new blood cells begins to decline, producing a measurable depression in blood counts. This threshold reflects the point where enough hematopoietic stem and progenitor cells are affected to cause noticeable cytopenias, especially affecting white cells first.

As the dose increases, the depression becomes more pronounced and lasts longer, with larger exposures causing more severe marrow suppression. Very high whole-body doses, such as around 10 sieverts or 25 sieverts, would produce acute radiation syndrome with dangerous marrow failure and high mortality risk, which is well beyond the simple depression of blood counts.

So the best, minimal dose that can depress blood counts is about 0.25 sievert, reflecting the hematopoietic system’s sensitivity to radiation.