PRINCIPLES OF TOXICOLOGY - Biology East Borneo

22.4 CASE STUDIES 545TABLE 22.3 Margins of Safety at Forest SitesLocationMargin of SafetySite 1 296Site 2 209Site 3 61Site 4 183• Biological hazards such as AIDS, tuberculosis, and hepatitis• Ergonomic problems• Antineoplastic drugs• Formaldehyde• Waste anesthetic gases (nitrous oxide and fluorinated hydrocarbons)• Ethylene oxide (a gas used to sterilize certain instruments)In a survey of one hospital operating suite, nitrous oxide was monitored with a miniature infraredanalyzer (MIRAN). The TLV ® for nitrous oxide is 50 ppm 8-h TWA and the NIOSH REL is 25 ppm(operating procedure TWA). In the operating rooms themselves, the concentrations were kept wellbelow these levels. Only once, and for a very short period, did the level rise above 50 ppm. The hoseconnected from the nitrous oxide wall mount receptacle to the gas-mixing unit was accidentally kickedfree. Concentrations in the room quickly went to 100 ppm. As soon as the levels went up, the hose wasnoticed, replaced, and levels quickly returned to previous levels. The low levels in the operating roomswere expected, since they have 15–17 air changes per hour, and 100 percent of the air is fresh, sterileair. However, in the recovery rooms, the situation was not as well controlled.In the recovery room, patients who were anesthetized exhale nitrous oxide–laden breath. Sincenitrous oxide is not very soluble in blood, it quickly comes off from the blood in the lungs. Nursesmust frequently bend over the patient’s head to talk with them and assess the conscious level of thepatient, which places their breathing zone in the breath exhalation area of the patient. Concentrationsin the exhaled breath of the patients were measured up to several thousand parts per million.General recommendations were made to minimize exposure by bringing as much fresh air aspossible into the recovery room. Because patients already were complaining of feeling cold (in partdue to the anesthesia), it would be expensive to condition all the air that enters the room during thewinter. Another recommendation was to locate a local exhaust duct near the head of the patient toremove nitrous oxide from around the head. A difficulty is that many patients just coming out fromunder sedation would not understand and recognize a duct near their heads, which could causeadditional stress.In the hospital operating room example an additional condition that allowed for low-level exposurewas that nitrous oxide was delivered by placing a tight-fitting mask over the patient’s mouth and nose, oranesthetic could be delivered via intra-tracheal intubation, thus minimizing leakage at the point of delivery.Dental operations do not have this luxury. A study of over 30 dental offices revealed that levels ofnitrous oxide exceeded 50 ppm by wide margins. Several reasons were observed. The pipes in thenitrous oxide delivery system frequently leaked, the mixing/delivery units were overpressurized andleaked, and the scavenging system was overwhelmed by the delivered volume of gas and exhalationof the patient.The recommendations to reduce exposure include minimizing the use of nitrous oxide to patientsthat truly need the sedation it affords; its routine use should be eliminated. The delivery flow ratesshould be reduced to the minimal effective flow rate (which varies from one patient to another). A damshould be placed in the back of the mouth to minimize the nitrous oxide, which is exhaled through themouth. The scavenger flow rate should be set at a level, which would effectively remove nitrous oxideas it is exhaled. The scavenger flowrate should also maintain a slight negative pressure inside the