The rest of the population is spread over smaller cities and rural farm lands. Because of the numerous mountains, it used to take much longer to get around the country, but tunnels have made it much quicker. You can now drive from Seoul in the north to Jinju in the south in about five hours.
I believe the largest challenge people face is the yellow dust pollution that clouds the air in so many parts of this country. These fine and ultrafine dust particles can be so bad at times that you can see the haze in the air. This pollution is said to be carried over from China by the wind and is worse in the winter and spring time. Depending on how the air pollution affects you, it can have long-term and short-term impacts on your health, prompting breathing and skin problems. Even in my small town, far away from the city, we are still plagued with poor quality air. The other main problem in Korea is the uneven distribution of population, leading to over-population in Seoul and underpopulation elsewhere in the country.
For air pollution, Korean officials urge everyone to wear masks when they are outside, especially if they have any pre-existing conditions or if the air quality is bad. Most homes, classrooms, and public spaces also have air purifiers that circulate clean air into the rooms.