The Boston Marathon is a remarkable facet of the city's culture and history for a number of reasons, but the sheer scope of the world's oldest marathon, now in its 123rd year, is impressive in itself.

The thousands of qualified runners rejected each year

Data visualization: Lisa Williams/WGBH News

As marathon running has ballooned in popularity in recent years, the Boston Marathon has been unable to accept each applying runner because of size restraints — even if they meet the qualifying time based on their age and gender.

The Boston Athletic Association (B.A.A.), the group that puts on the marathon each year, is only able to accept approximately 23,000 qualified runners into the race each year. That means that for the past few years, thousands of qualified runners have not been accepted due to the size cap. That number reached its highest yet for the 2019 marathon, when 7,384 qualified runners were rejected even after meeting their standard.

The B.A.A. opens registration to qualified runners in waves. Those who are 20 minutes faster than their qualifying standard are accepted and able to register first, followed by those who are 10 minutes faster, and then five minutes faster. Then, the association determines how much faster their runners have to be than the qualifying standard in order to best fit that 23,000 runner limit. When runners submit their qualifying marathon time, they won't know if they'll be running the marathon in April until they hear back from the B.A.A.

For the 2020 marathon, the B.A.A. has announced that they will change qualifying times to be five minutes faster across all ages and genders "as the number of marathoners who have submitted applications to run the Boston Marathon has increased significantly during the most recent two registration years."

Data visualization: Lisa Williams/WGBH News

Here's what this essentially comes down to: The greater the number of runners who qualify for the Boston Marathon, the faster each person has to be in order to actually be accepted.

To give all the non-marathoners out there an idea of just how difficult it's been to be accepted to the Boston Marathon for the past few years, we calculated the average speed at which a runner would need to go in their qualifying time in order to be accepted each year. The most common age of Boston Marathon runners is 45 years old, so we used the accepted qualifying times of 45-year-old men and women.

While every runner has a different running style, and most marathoners certainly don't run at a completely constant pace throughout the race, it hits home just how much each second counts when it comes to who gets to reach the starting line.

Remember: This is the absolute slowest runners would have to have been in order to be accepted.

The toll of the temperature

Data visualization: Lisa Williams/WGBH News

The forecast can have a big impact on a race's outcome. And while the harsh rain and wind of the 2018 marathon stands out in recent memory of tough conditions runners had to combat, the heat of 2004 and 2012 shouldn't be forgotten, either.

Hot temperatures are the biggest weather factor runners need to look out for, said Dan Fitzgerald, the co-founder of Heartbreak Hill Running Company and the founder and coach of the Heartbreakers, a U.S.A. Track and Field certified running club.

According to Fitzgerald, the ideal temperature range for running the Boston Marathon would be between 45 and 50 degrees, reflected as a blue band in the chart above.

Temperatures have typically been warmer than that over the years. While every runner is different, Fitzgerald said that high temperatures are particularly challenging for runners who have been training in cooler places like Boston.

"Nothing hurts like the heat," Fitzgerald said.

The big bucks of apparel sponsors

Data visualization: Lisa Williams/WGBH News

The number of marathoners isn't the only thing that's increased over the past few years. The apparel companies that sponsor the biggest marathons across the country have all seen upticks in annual revenue.

Adidas, which has been the Boston Marathon's official apparel sponsor for 31 years now, is the second-largest of the four, raking in $24.68 billion in 2018.

Boston-based New Balance saw revenues of $3.8 billion in 2016, $4 billion in 2017 and $4.1 billion in 2018, a spokeswoman told WGBH News. New Balance sponsors the New York City Marathon — the biggest in the country — as well as the Detroit Marathon.

"[W]hile we don't sponsor the Boston Marathon, we guerrilla market around it," president and CEO Robert DeMartini said in a June 2018 interview.

The global reach of winners' homes

Data visualization: Lisa Williams/WGBH News

Since 1985, all but two Boston Marathon winners have been from outside the United States for both the men's and women's divisions.

When she won the women's race in 2018, Desiree Linden became the first American woman to do so in 33 years. Mebrahtom "Meb" Keflezighi won the men's race in 2014, the first American man to win since 1983.

Last year's male winner, Yuki Kawauchi, is from Japan, the first runner from Japan to win the Boston Marathon since 1987.

Between 1991 and 2012, all but three of the male winners have been from Kenya; women from Kenya have won six of the past 10 years.