You can imagine the advisement late one night on T.V.: “Did you or a loved one recently suffer from deep vein thrombosis or a pulmonary embolism after a long flight? If so, call the law offices of . . .”
Implausible? Far-fetched? Consider that the associated risks of deep vein thrombosis (DVT) and/or a pulmonary embolism with long periods of confined seating have been known for decades. The airlines are acutely aware of this risk, understand that exercise and movement mitigate its potential impact, and yet continue to reduce seat size and pack the cabin like an overstuffed cattle wagon. Little is done to brief the passengers on this danger or to provide enough leg room so that the passenger can undertake the recommended exercises to avoid this health concern.
The Federal Aviation Administration (FAA) has no regulations concerning minimum seat space. Nevertheless, the FAA is aware of the risks of DVT. According to its pamphlet entitled, “Deep Vein Thrombosis and Travel” , a clot or thrombosis can form in the deep vein of a leg from long periods of sitting and inactivity. A pulmonary (lung) thrombosis can form for the same reason. Only certain people are susceptible to these conditions, but it is generally believed that restriction in blood flow caused by sitting is causally related to the condition. The common symptoms are swelling and redness in the affected leg. Severe chest pain and difficulty breathing are signs of a pulmonary thrombosis.
A storm over passenger seating has been brewing for a while. It is no secret that such confined spaces are making passengers angry. During the week of August 2014, for example, CNN reported a series of passenger skirmishes over reclining seats. One of the skirmishes led to the diversion of a Miami to Paris bound American Airlines’ flight and criminal charges against the unruly passenger. 
As reported in USA Today on September 24, 2014,  the data supports that airline seating is shrinking. Seat pitch (or the distance between seats, front and back) has shrunk on average anywhere from two to five inches among the major carriers over the last several decades. Likewise, seat width has been trimmed a couple of inches as well. Id. Try opening your laptop sometime in coach and you will be lucky to hit control-alt-delete to get it going, let alone nurse a cup of coffee at the same time!
FlyersRights.Org, an airline passengers’ rights group, started a Petition for a Passenger’s Bill of Rights which included the requirement of minimum seat space.  Additionally, Senator Charles Schumer of New York proposed legislation this January concerning this very topic.  And let us not forget the Knee Defender, a handy product with a key, no less, and now forbidden, which stopped the seat in front of you from fully reclining. 
On February 11, 2016, an Air Bus patent application was published which illustrated bench seating:
Just imagine how the middle seat’s occupant will feel with this configuration! There is no question that passengers are feeling slighted about their cramped space. But what are the health risks for such confined seating?
For years, the possible connection between seat space and adverse medical conditions, such as thrombosis, was commonly known as “economy class syndrome” subsequent to study published in 1988, entitled Air Travel and Thrombosis episodes: the economy class syndrome. Lancet: 1988, 2: 497 - 498.
In 2002, the World Health Organization (WHO), recognizing the long association between air travel and venous thrombosis, and the lack of detailed studies, recommended specific epidemiological studies be undertaken to quantify the risk factors for Venous Thromboembolism (VTE) (European convention for both DTV and pulmonary embolisms) associated with air travel.  Phase I of that study, which was a collaboration of the WHO, the International Civil Aviation Authority (ICAO), and the International Air Transport Association (IATA), was finally published in 2007. The Phase I Final Report confirmed that the risk of VTE approximately doubled after a long-haul flight (>4 hours). The risk also applies to other forms of travel (such as car, bus, or train) and the risk increased with the duration of the travel and multiple flights. Other factors, such as extremes of height, obesity, use of oral contraceptives, and the presence of pre-existing abnormalities contribute to the risk. The relationship between high altitude and VTE (through hyperbolic chamber testing) was not established.  The study concludes that passengers should be informed of the risk and that methods of identifying effective preventative measures should be studied in the next report, i.e. Phase II. Unfortunately, Phase II has not received funding.
Certainly the increased risk associated with height and weight should factor into airline seating space. Several airlines now provide information to their passengers on DVT. For example, United Airlines offers its passengers the following exercises, including ankle circles, foot pumps, shoulder roll, and the knee to chest stretch: 
How exactly is a passenger supposed to find space to undertake these exercises if their knees are already constrained by the seatback in front of them? And let us not forget the potential emotional impact that some individuals feel when confined into small spaces!
So buckle up and do not forget to wear the recommended support stockings for flights over four (4) hours.
 Written by: Wolbrink, M.D., Alex, and Prepared by: Federal Aviation Administration Civil Aerospace Medical Institute, Pilot Safety Brochures, AM-400-03/2, “Deep Vein Thrombosis & Travel,” https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/DVT_07182005.pdf [last accessed 3/28/16].
 http://www.cnn.com/2014/08/29/travel/airline-passenger-seat-recline/ [last accessed 4/1/16].
 http://www.usatoday.com/story/travel/columnist/mcgee/2014/09/24/airplane-reclining-seat-pitch-width/16105491/ [last accessed 4/1/16].
 http://new.flyersrights.org/uncategorized/87/ [last accessed 4/1/16].
 http://www.nytimes.com/2016/02/29/travel/shrinking-airline-seats.html?_r=0 [last accessed 4/1/16].
 http://www.huffingtonpost.com/2014/08/28/knee-defender-ban_n_5723136.html [last accessed 4/1/16].
 TRAVEL, GLOBAL HAZARDS OF. "WHO research into global hazards of travel (WRIGHT) project." (2007). http://www.who.int/cardiovascular_diseases/wright_project/phase1_report/WRIGHT%20REPORT.pdf [last accessed 4/1/16].
 https://www.united.com/web/en-US/content/travel/inflight/health.aspx [last accessed 4/1/16].