well these stages of poisioning are the percentange of the saturation of the blood with carbon monoxide. if being more specifically it's percentage of the blood's heme that has been subjected to chemical reaction with CO to form carboxyhemoglobin(COHb), so this compound incapable to saturate body's tissues with oxygen and causes oxygen starvation and consequently the brain hypoxia and death. being in conidition of small percentage(20%) of COHb saturation for long time can lead to certain symptoms like shortness of breath as body tries to compesate oxygen shortage or to adapt to it,
so a degree how it's painful to die from CO depends on time of exposure and gas concentration, small concentrations with long times of exposure allowing the body to adapt and it gves flue-like symptoms of poisoning. as example if gas concentration per volume is 1000ppm then you need several hours to die and it's enough to experience all variety of symptoms painful they're or not. converselly being exposed to high concentrations(10,000ppm or more) for short time leads to rapid COHb saturation with rapid loss of conscience allegedly without symptoms as body is incapable to compensate for oxygen shortages so having no choice but to drop dead instead. there's CO poisoning charts with given ppm and corresponding time of death and symptoms for each concentration, surprisingly, high concentration don't have symptoms of lower ones on the line, just check out some CO poisoning tables in google