By the time you make it to the airport, check your bag, make it through the security line, and actually set foot on the airplane, you may not be in the mood for more than a polite hello to the flight attendant who welcomes you onboard. While you may not be able to manage more than a cursory glance and brief smile at the flight attendant while you struggle aboard with your bags, there's a good chance that the flight attendant is taking a much closer look at you.

According to a recent informal Quora survey of flight attendants, they size up their passengers from the moment they enter the plane door. They are looking for information about their passengers, ranging from potential risks to potential help and they do it all in a blink of an eye.

Flight attendant Amar Rama likes to keep an eye out for sick passengers, noting that while the passenger may want to travel, if a serious medical issue does arise, it's far better to be on the ground than 30,000 feet in the air. "Flight attendants are all trained in CPR, Automated External Defibrillators, basic first aid emergencies," she wrote. "But we cannot diagnose you nor have the expertise, experience or treatment as doctor."

Flight attendants aren't just looking for germs, though. According to Sjaak Schulteis, who worked for Lufthansa for 30 years, flight attendants are also on the lookout for passengers who are impaired by drugs or alcohol. "If a guest coming aboard is drunk or intoxicated by any drug, it can happen that he or she is not allowed to enter the plane," wrote Schulteis. "The first impression is often the right one, and we do refuse passengers who might be a danger for the safety of that flight."

If a problem passenger does make it onboard, the flight attendants might need back-up—and they might turn to a passenger for assistance. When passengers board, the crew might be looking for someone who can help them in a pinch. "If I see someone who is muscular, powerful, strong, physically fit, I memorize his/her face and make a mental note of where they are sitting," wrote Janice Bridger, a flight attendant of 27 years. "I consider this person a resource for me."

Flight attendants also keep their eyes open for airline employees who may be off-the-clock, but can jump in to help in a pinch. "They know how to handle the situations as well as I, and are trained to become an instant 'team member,' fitting right in immediately if needed," wrote Bridger.

If you're thinking that this sounds like a lot of work for a flight attendant, you're right. Bridger wrote, "When you consider that I have approximately 3-4 seconds to make that passenger feel welcomed and comfortable, and then also assess them for all of the potential that they bring with them onto the plane... well, it can require a lot of focus." All the more reason to be extra polite to your flight attendants when you travel.