Boeing tested cabin altitude on 502 volunteers in a sealed chamber, then chose 6,000 ft

According to WIRED UK, Boeing commissioned a study at Oklahoma State University that began in October 2002. Over seven months, 502 volunteers went into an airtight hypobaric chamber fitted with 12 economy seats spaced 34 inches apart, the typical US spacing at the time. Each group spent about 20 hours inside with films, a sleeping period and meals, but no alcohol. Each group was held at a different simulated altitude, from ground level up to 2,438 m.

The study found the cabin should be pressurised as if the plane were flying at 1,829 m, or 6,000 ft. Boeing's director of differentiation strategy, Blake Emery, said that of all flight features, pressurisation is probably the one most responsible for reduced physical symptoms. The article says any higher and the oxygen reaching the bloodstream drops by 4%, and says earlier Boeing planes were pressurised to up to 8,000 ft.

Emery says the lower cabin altitude was applied across the 787 Dreamliner and 777X, and claims it reduces jet lag. He also notes that, with cabin space fixed, airlines and makers mostly have to find gains in how passengers perceive comfort.

Rather than guess, run a seven-month chamber experiment with 12 real seats and hundreds of volunteers, and let the results set the engineering target.

Cabin pressure is a setting rather than a space cost, so it improves comfort without taking floor area that airlines sell.

The test replicated a real long flight, with meals, sleep and 12 seats at typical spacing, so the symptoms measured were realistic.

Testing several altitudes in separate groups let Boeing identify an optimum instead of a single guess.

A reading of 4% less oxygen at higher altitudes gave a concrete threshold to design against.

For comfort-versus-cost trade-offs, a controlled human experiment can beat intuition, especially when a single parameter drives many symptoms.

WIRED describes the pressurisation change as one of Boeing's biggest overhauls in recent years, and says Boeing applied it to the 787 and 777X. The jet-lag benefit is Emery's own claim; the article gives no independent measurement.