This is the 808th most frequent Korean word.
실내
"Indoors" or "interior."
Here, '실내' is used to describe the air within an enclosed space, emphasizing maintaining a clean indoor environment.
실내 공기가 깨끗하게 유지되도록 창문을 열어 환기를 시켜야 합니다.
To keep the indoor air clean, you should open the windows to ventilate.
In this sentence, '실내' defines a general indoor setting where the action of smoking is mentioned as restricted.
실내에서 담배를 피우는 것은 금지되어 있습니다.
Smoking indoors is prohibited.
In this sentence, '실내' refers to a physical space inside a building where an activity like exercising can be performed, specifically indicating an indoor context.