Should you approach a check-in desk of the budget US carrier Southwest Airlines, try to look narrow. Those deemed too wide for their seat are now to be charged for a neighbouring one as well. Now wedged in a public relations fiasco, Southwest says it is merely making explicit a long-standing policy for passengers who encroach on the space of those seated around them.

It is tempting to see the issue as a storm in a super-sized cola cup (with a burger and large fries please), but obese Americans are not the only ones being squeezed by the airlines. The average width of economy seats on the ten most popular international airlines is less than 18 inches, though those on SAS were a spacious 21 inches.

Seats as narrow as 15 inches were found by a recent Civil Aviation Authority (CAA) study looking at the safety implications of UK aeroplane layouts. While Britons are not as burly as their American cousins, more than a third of British women and a fifth of men would not fit their hips into such a small seat. More than 5% of British women would not even be able to squeeze between the armrests of an average economy seat.

How wide is your economy seat?
# Lufthansa 17 inches
# British Airways 17.25 inches
# Swissair 17.3 inches
# United Airlines 18 inches
# SAS 21 inches

How wide are your hips?
# 50% of British women 13.9 inches plus
# 36% of British women 15 inches plus
# 5% of British women 19 inches plus
# 1% of British women 21 inches plus
Sources: IATA, System Concepts

Seats on UK trains, coaches and buses must be at least 17 inches across according to government regulations, however no similar stipulation applies to the UK’s airliners. And it’s not only comfort at stake. Those crammed into their seats before take off may find it difficult to escape in an emergency, according to British ergonomists evaluating the current rules.

A person struggling to leave a narrow seat may also hinder the evacuation of other passengers – a problem that not necessarily avoided by forcing large travellers to buy two seats. The CAA’s Airworthiness Notice 64 (AN64), which sets the standard for seat spacing on UK aircraft, does stipulate a minimum distance of 26 inches between your seat back and the seat in front.

This so-called seat “pitch” – how much legroom a passenger enjoys – has been the focus for the recent debate over deep vein thrombosis (DVT). However, seat width has also been suggested as a contributory factor to the potentially fatal blood clotting condition. While squeezing yourself into a too-small seat can cause the sort of “tissue compression” that could restrict blood flow and trigger DVT, even slim-hipped passengers may be at risk.

“[A narrow seat] restricts the opportunity for passengers to change posture, not only because of the limited space but also because of the disturbance it may cause the adjacent passenger,” says the CAA survey. ]