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Below the windows, ahead of the seat, there are small grilles. Look: Boeing 787
Source: Airliners.net

What are those for?

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The answers to this question indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.

The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.

US patents # 6129312 and # 5137231 claim systems for combining the cabin air circulation vents with the depressurization relief vents. From the '9312 patent:

An air return grille mounted below the passenger cabin sidewall comprises openings to allow return airflow from the air conditioning system. The grille openings are covered by a single thin baffle with cutouts within its periphery that slip over and are held in place by fasteners on the molded grille. During normal operation, the thin baffle restricts flow through a small opening in the grille. During a sudden decompression of the cargo compartment, the baffle is dislodged from the fasteners. This allows the airflow to pass through all of the molded grille openings in order to achieve rapid pressure equalization.

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    $\begingroup$ The panels are usually referred to as "dado panels". As an example of what happens when an explosive decompression is too violent for the dado panels to equalise the pressure (and protect the floor structure), see AA Flight 96. $\endgroup$ – Lightsider Mar 11 '16 at 9:47
  • $\begingroup$ @Lightsider will that blow away passengers legs? $\endgroup$ – Firee Jul 7 '17 at 6:22
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The grilles are almost certainly meant to equalize pressure between the cabin and cargo bay, particularly in case of rapid decompression. As far as I know, the design feature stems from two consecutive accidents with the DC-10, American Airlines 96 on 12 June 1972, and Turkish Airlines 981 on 3 March 1974. Though the cause of both of these decompressions was a poorly-designed (and poorly-maintained) cargo door, in both cases, the real damage was caused by the collapse of the cabin floor, which severed control cables, and on Turkish Airlines, broke hydraulic lines. After these accidents, the grilles were added to equalize pressure and prevent the floor from collapsing. The floor itself was also strengthened.

If I remember correctly, one of these accidents was the subject of an episode of Air Crash Investigation.

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Air Crash Investigation S7E1 "Shattered in seconds" about China Airlines flight 611 on 2002-05-25 (decompression due to fuselage rupture caused by a faulty repair after a tail strike on 1980-02-07) mentions dado panels and an accident with a Turkish Airlines flight in 1974 (together with reconstruction footage of that accident), and it is said that the Turkish Airlines accident led to a NTSB recommendation about adding more dado panels to the cabin. All of this is in sum a good indication that there also is an episode about that particular crash.

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