PRINCIPLES OF TOXICOLOGY

TABLE 22.3 Margins of Safety at Forest Sites
Location Margin of Safety
Site 1 296
Site 2 209
Site 3 61
Site 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)
22.4 CASE STUDIES 545
In a survey of one hospital operating suite, nitrous oxide was monitored with a miniature infrared
analyzer (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 well
below these levels. Only once, and for a very short period, did the level rise above 50 ppm. The hose
connected from the nitrous oxide wall mount receptacle to the gas-mixing unit was accidentally kicked
free. Concentrations in the room quickly went to 100 ppm. As soon as the levels went up, the hose was
noticed, replaced, and levels quickly returned to previous levels. The low levels in the operating rooms
were expected, since they have 15–17 air changes per hour, and 100 percent of the air is fresh, sterile
air. 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. Since
nitrous oxide is not very soluble in blood, it quickly comes off from the blood in the lungs. Nurses
must frequently bend over the patient’s head to talk with them and assess the conscious level of the
patient, which places their breathing zone in the breath exhalation area of the patient. Concentrations
in 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 as
possible into the recovery room. Because patients already were complaining of feeling cold (in part
due to the anesthesia), it would be expensive to condition all the air that enters the room during the
winter. Another recommendation was to locate a local exhaust duct near the head of the patient to
remove nitrous oxide from around the head. A difficulty is that many patients just coming out from
under sedation would not understand and recognize a duct near their heads, which could cause
additional stress.
In the hospital operating room example an additional condition that allowed for low-level exposure
was that nitrous oxide was delivered by placing a tight-fitting mask over the patient’s mouth and nose, or
anesthetic 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 of
nitrous oxide exceeded 50 ppm by wide margins. Several reasons were observed. The pipes in the
nitrous oxide delivery system frequently leaked, the mixing/delivery units were overpressurized and
leaked, and the scavenging system was overwhelmed by the delivered volume of gas and exhalation
of the patient.
The recommendations to reduce exposure include minimizing the use of nitrous oxide to patients
that truly need the sedation it affords; its routine use should be eliminated. The delivery flow rates
should be reduced to the minimal effective flow rate (which varies from one patient to another). A dam
should be placed in the back of the mouth to minimize the nitrous oxide, which is exhaled through the
mouth. The scavenger flow rate should be set at a level, which would effectively remove nitrous oxide
as it is exhaled. The scavenger flowrate should also maintain a slight negative pressure inside the