Yes, NO is very toxic. It is easy to combine with hemoglobin Hb in the blood, leading to blood hypoxia and paralysis of the central nervous system. The affinity of NO with hemoglobin is several hundred times to several thousand times that of CO ... NO2 is a yellow-brown gas with pungent smell, which is 4-5 times more toxic than NO. It will stimulate the respiratory system and lead to emphysema.