Which of the following isotopes is most dangerous to homo sapiens?

1. phosphorous-32
2. strontium-90
3. caesium-137
4. iodine-131

The radioactive strontium – 90 is most dangerous as it gets fixed in bones.and may remain in bones leading to bone cancer. It emits high energy beta radiations. Phosphorus – 32 also emit high energy beta radiations but they cannot penetrate human skin. Caesium – 137 (beta emitter) and iodine – 131 also pose danger to human health but not more than strontium 90.