Date: 202202
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Workload; Troubleshooting; Time Pressure; Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Hazardous Material Violation
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: During preflight of flight release we saw we had a deferred APU.  After receiving the preliminary weight and balance; Dispatcher sent us an ACARS; stating that we will be operating under supplemental Dry Ice procedures. my first officer; and I both agreed we didn't think we could do that. I then called our Dispatcher and asked him about it;  with his explanation which was that they were going to load the upper [cargo bin] with 952 kgs and the forward belly with 264kgs;  according to the dispatcher; that would keep us under the limits of the MEL (XX-XX-XX-A). I had the MEL open while talking with dispatcher; and I saw the numbers he was talking about. So; I agreed that we met the MEL limits.  We (the Crew); decided we would continue flight under the Dry Ice Supplementary procedures as outlined in QRH; with the exception of the APU usage.  After departure and while climbing to cruise altitude; I just kept having a feeling that wasn't the correct thing to do; so after TOC; I started talking with the FO about it; after about 15 minutes and looking at the MEL for the 5th time; I had finally found MY error.  The MEL; XX-XX-XX-A; does have maximum dry ice limits for the upper and forward cargo compartments; but; if I had read further down; it also has a maximum dry ice for ALL cargo compartments; basically max amount of dry ice allowed..which was 966 kgs..we had roughly 1200 kgs of dry ice on board.   This was my mistake; not the FO; and not the dispatcher; I failed to read and interpret the MEL correctly.  Once we landed in ZZZ1; I informed the off load crew to vent the aircraft before entering for offload. In my opinion; other than my failure to read all and interpret the MEL correctly; I believe this MEL; should have the maximum dry ice of 966 kg highlight a different color from the other limits.  Also; I believe the words ( Supplementary dry ice procedures are not allowed) written in bold letters in remarks sectionWhen something feels like it is incorrect; take the time to discuss the situation until it feels correct. I should have taken more time to look at MEL longer;   Also change the MEL to actually state; supplementary dry ice procedures not allowed


Second narrative: We had an APU deferred per MEL XX-XX-XX-A.  Shortly before departure we received an ACARS message that dry ice supplemental procedures would be in effect (not included on the release).  We then received our DG paperwork (nearly simultaneously) stating a significant amount of dry ice: 952.11 upper deck; 264.33 FWD lower deck.  The DG specialist informed me that some dry ice had been removed from the upper deck and placed in the lower forward compartment to comply with the MEL.  The Captain and I both questioned what was happening as we are both aware that dry ice procedures require running the APU on the ground.  The Capt called our dispatcher while I deployed the draeger monitors and inspected the Hazmat can.  The dispatcher indicated that the amounts in each compartment were under the limits per the MEL (they were) and that we were good to depart.  We both referenced the MEL; but all of us failed to recognize that the total dry ice exceeded the allowed amount.  Not until we reached cruise did we reread the MEL several times in more detail did we realize our mistake.  Upon landing; we notified ground personnel of the dry ice amount and need to vent the airplane prior to unloading (during blue light procedures due to lightning).  The Capt also called dispatch to inform them of our error.  On a side note; we attempted to write our ASAP reports while in the [flight crew lounge] while waiting for our next leg but the server appeared down.  After our next leg and subsequent rest was our first opportunity.[Flight crew stated] a late notification of dry ice supplemental procedures in conflict with our MEL during final cockpit preparation; combined with a misreading of the MEL dry ice limitations by all involved.[Flight crew also added] that both the Captain and I had a gut feeling that something was off.  Ultimately we should have conducted a more in depth and critical review of the MEL prior to departure.  However; considering all three of us missed the total dry ice limitation for a very simple MEL; I think it should be considered to somehow highlight the total restriction in the MEL table.  Also adding a note that 'Dry ice supplemental procedures are not authorized' under the Operations (O) notes would have been helpful.  This also impacts MELs XX-XX-XX-B and XX-XX-XX-XY.


Synopsis: Air Carrier flight crew reported during cruise that they had exceeded supplemental dry ice weight limitation due to misinterpreting the assigned MEL restrictions. Upon destination arrival flight crew informed the offload crew to follow venting procedure before cargo offload.