RedactedNarrative: On  07/08/2019, at approx. 0315 hours (MST), my partner and I were dispatched to a report of possible carbon monoxide (CO) poisoning aboard the houseboat Arrivaderci in West Canyon (Lake Powell).  We learned from NPS dispatch that Classic Air Medical had been dispatched to the scene and had arrived there at approx. 0320.  Classic was requesting we respond and bring the blood CO monitors we carry aboard our vessels as Classic does not carry these devices.
  
  We arrived on scene at about 0430.  By this time, Classic Air had transported two patients to Banner Page Hospital in Page, AZ for evaluation of CO poisoning.  Both patients were female, aged 39 and 40 years.  The Classic Air Medical flight medic had been left on scene to monitor the status of the vessel's other passengers.
  
  Upon our arrival, Ranger and I were directed by the flight medic to begin measuring CO levels (spCO) with our two Massimo carboxyhemoglobin meters and distributing O2 at 15L/min to several patients.  We only carried two O2 tanks.  O2 administration was done on an ad hoc basis until the patient felt "improved" and then the mask was switched out and the O2 was administered to another patient.
  
  The highest measurement of spCO taken was 22.  Other values measured were: 18 (0433 hours); 20 (0437); 19 (0438); 15 (0439); and 14 (0435).  Other values were not recorded.
  
  Per the flight medic's instructions, all patients were kept outside the vessel and breathing fresh air.  All improved (lessened nausea and headaches) during our time there.
  
  - Just before 0230 hours, SUBJECT1 and Subject2 awoke with headaches.  Both went outside.  SUBJECT1 had smelled exhaust inside the vessel and had turned off the generator.  Afterwards, he opened some of the windows in the vessel's aft parlor area.
  - At about 0230 hours (estimated time), SUBJECT3 had awoken to use the vessel's head.  While walking down the hallway, she experienced a syncopal episode. Her falling to the floor awoke other passengers, including SUBJECT4 who said that SUBJECT3 was "unresponsive" for about 10-15 minutes.
  - Sometime around the same time, Subject5 also experienced a syncopal episode and was also unresponsive for about 5 minutes. Both women and all the remaining passengers were evacuated from the boat to a location on shore.
  - A number of the passengers were experiencing headache, nausea, and vomiting.
  - SUBJECT4 contacted NPS dispatch via VHF marine band 16.
  - The vessel was equipped with CO alarms in the bedrooms.  One or more of these had gone off last year while the generator had been running.  The generator was switched off and the alarms reset during that incident.
  - None of the CO detectors in each of the vessel's bedrooms activated.  All had power and seemed to test ok.  A CO detector in the main parlor area (in the area likely first infiltrated by gasses) was without a battery.
  
  Ranger and I conducted an investigation to locate the cause(s) of the CO poisoning.  The vessel's generator was located in an aft engine compartment.  The exhaust port was located on the side of the vessel at the stern, approximately 1 foot above the waterline.  The parties had moored a ski boat closely against the houseboat at this location.  It seems likely that exhaust gases were trapped in this area due to the area being generally blocked from ventilation/circulation by the ski boat.  As they accumulated, the gasses infiltrated the vessel, starting with the aft area where the main parlor was located (and where 6-8 of the passengers were sleeping).
  
  I spoke with SUBJECT4 and he agreed to consider modifying the exhaust system to vent well above the upper deck of the vessel.  He also agreed to have the CO detectors in the bedrooms checked.
  
  The flight nurse gave continuing care instructions to all patients and we departed the scene when classic did at about 0603.
  
  I learned later that both of the female patients had improved at the hospital in Page and were expected to be released later that day.
  
  At approximately 1500 hours that day, I was advised by NPS dispatch that a subject at Pheonix Poison Control wanted to reach passenger Subject2 who was 23 weeks pregnant.  I spoke with subject by telephone.  He said that Poison Control advises that pregnant females be checked out by their Ob/Gyn doctor after being exposed to CO as the fetus may take up more of the compound than the mother.  I called SUBJECT4, SUBJECT1 and Subject2 and left each of them voice mails to this effect.  I spoke with SUBJECT4 three days later and he confirmed they had received the messages.