Mass carbon monoxide poisoning: Clinical effects and results of treatment in 184 victims

“An epidemiologic and clinical investigation of 184 persons exposed to toxic levels of carbon monoxide (CO) in a public high school has been carried out. Exposure to 500 ppm of CO for periods up to 150 minutes resulted in carboxyhemoglobin (COHb) levels as high as 30% and symptomatic illness in 87% of persons exposed before the possibility of mass CO poisoning was recognized.”

Mass carbon monoxide poisoning: clinical effects and results of treatment in 184 victims – PubMed (nih.gov)