Narrative:

Approximately 2 hours into the flight; I was contacted by our purser. She asked me if we were doing any turns or climbs or descents because she and a few other flight attendants were feeling dizzy and nauseous. I told her no; we had been in straight and level flight for the last two hours. I told her I'll try to increase the air quality in the cabin; so I turned off the right recirculating fan. I also asked her if she or any of the other flight attendants smelled anything strange; like wet dirty socks. We also checked the cabin altitude to make sure that the cabin was not climbing. It was stable at around 8500 ft; and made sure there were no ozone areas in the [weather] map. A few minutes later; [the flight attendant] contacted me again to let me know the flight attendants were getting worse; both the ones in the front of the airplane and in the back. That they were almost incapacitated. I asked her to wake up our international relief officer (international relief officer); and immediately contacted dispatch through satcom and requested a patch with medlink.[the first officer] went through the entire cabin and could not smell any fumes or anything unusual. But he did say some flight attendants looked ill. One of them reported blurred; narrow vision; another had teary eyes and was sweating. I had the purser come to the cockpit to listen what [to] the doctor had to say; as we went through the symptoms. The doctor recommended that some; but not all of the flight attendants take a medication called diphenhydramine to counter the symptoms. And he also recommended that we land as soon as possible. As a side note; the medication was not in the med kit the doctor said it should be. It was in the cockpit med kit. So his information needs to be updated/corrected.the decision was made to return to [the departure airport] because they have excellent medical facilities; longer runways; and some of the passengers originated from there. It was about a two hour flight to return from that point; and we were still looking at an overweight landing. We requested a turn back for medical reasons. We got immediate priority without requesting it.I had [the purser] pass her purser duties to [another flight attendant]; and directed that her and 2 other flight attendants with the worst symptoms take the pills. That gave us a minimum crew of 5 fas (flight attendants). We checked their status every 10 minutes; and told them to let us know as soon as possible if they got worse; in case we needed to divert somewhere closer. Most of them reported feeling better; especially after taking oxygen. They did measure their oxygen levels with the oxygen meter in the quick response kit. Initially; their O2 levels were unusually low; in the mid-80s.we asked ATC for lower altitudes for two reasons; lower cabin altitude; and higher fuel burn because we were projecting an overweight landing. On the way; we reviewed the smoke and fumes elimination; ozone; bio hazard and overweight checklists. Only the overweight landing checklist could apply. We did have an overweight landing (+/-4;000 lbs). That was necessary due to the nature of the emergency; we could not remain in the air longer to burn more fuel. Autobrakes disconnected on landing and manual braking was required. The left main gear temperatures were reading 4s by the time we exited [the runway]. [It was] a short taxi to the remote parking stand; and it took a few minutes for the ground personnel to install wheel chocks. By then; the left main landing gear temperatures had climbed to 9s. The left rear tire fuse melted and the tire deflated. Fire department; medical and mechanic personnel met the airplane and took care of the situation from there.flight attendants and some passengers were taken to the airport clinic. They felt ok and released shortly after. We found out next day that all flight attendants tested positive for CO2. Their symptoms during the flight verified that fact.

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Original NASA ASRS Text

Title: B767 flight crew reported a return to departure airport after several flight attendants and some passengers became lightheaded; dizzy; and nauseous. The flight attendants later tested positive for CO2.

Narrative: Approximately 2 hours into the flight; I was contacted by our purser. She asked me if we were doing any turns or climbs or descents because she and a few other flight attendants were feeling dizzy and nauseous. I told her no; we had been in straight and level flight for the last two hours. I told her I'll try to increase the air quality in the cabin; so I turned off the Right Recirculating Fan. I also asked her if she or any of the other flight attendants smelled anything strange; like wet dirty socks. We also checked the cabin altitude to make sure that the cabin was not climbing. It was stable at around 8500 ft; and made sure there were no Ozone areas in the [weather] map. A few minutes later; [the Flight Attendant] contacted me again to let me know the flight attendants were getting worse; both the ones in the front of the airplane and in the back. That they were almost incapacitated. I asked her to wake up our IRO (International Relief Officer); and immediately contacted Dispatch through SATCOM and requested a patch with Medlink.[The First Officer] went through the entire cabin and could not smell any fumes or anything unusual. But he did say some flight attendants looked ill. One of them reported blurred; narrow vision; another had teary eyes and was sweating. I had the Purser come to the cockpit to listen what [to] the doctor had to say; as we went through the symptoms. The doctor recommended that some; but not all of the flight attendants take a medication called Diphenhydramine to counter the symptoms. And he also recommended that we land as soon as possible. As a side note; the medication was NOT in the med kit the doctor said it should be. It was in the cockpit med kit. So his information needs to be updated/corrected.The decision was made to return to [the departure airport] because they have excellent medical facilities; longer runways; and some of the passengers originated from there. It was about a two hour flight to return from that point; and we were still looking at an overweight landing. We requested a turn back for medical reasons. We got immediate priority without requesting it.I had [the Purser] pass her Purser duties to [another flight attendant]; and directed that her and 2 other flight attendants with the worst symptoms take the pills. That gave us a minimum crew of 5 FAs (Flight Attendants). We checked their status every 10 minutes; and told them to let us know ASAP if they got worse; in case we needed to divert somewhere closer. Most of them reported feeling better; especially after taking oxygen. They did measure their oxygen levels with the oxygen meter in the quick response kit. Initially; their O2 levels were unusually low; in the mid-80s.We asked ATC for lower altitudes for two reasons; lower cabin altitude; and higher fuel burn because we were projecting an overweight landing. On the way; we reviewed the Smoke and Fumes Elimination; Ozone; Bio Hazard and Overweight checklists. Only the Overweight Landing checklist could apply. We did have an overweight landing (+/-4;000 lbs). That was necessary due to the nature of the emergency; we could not remain in the air longer to burn more fuel. Autobrakes disconnected on landing and manual braking was required. The left main gear temperatures were reading 4s by the time we exited [the runway]. [It was] a short taxi to the remote parking stand; and it took a few minutes for the ground personnel to install wheel chocks. By then; the left main landing gear temperatures had climbed to 9s. The left rear tire fuse melted and the tire deflated. Fire department; medical and mechanic personnel met the airplane and took care of the situation from there.Flight attendants and some passengers were taken to the airport clinic. They felt OK and released shortly after. We found out next day that all flight attendants tested positive for CO2. Their symptoms during the flight verified that fact.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.