Which of the following factors is NOT typically associated with the increase of upper respiratory infections in colder months?

• A. Indoor crowding
• B. Lower humidity levels
• C. Exposure to sunlight
• D. Increased indoor gatherings

Answer: (C)

In winter, the rise in upper respiratory infections is driven by how people spend time indoors and how the air behaves there. More indoor crowding and increased indoor gatherings raise the chances of viruses passing from person to person, while lower humidity from indoor heating dries the mucosal surfaces and can help some viruses survive longer in the air and on surfaces. All of this makes transmission more likely. Exposure to sunlight, on the other hand, isn’t a driver of this winter surge. In fact, sunlight can support immune function through vitamin D and, where UV light is involved, can help inactivate some viruses. So sunlight exposure is not typically linked to the winter increase in URIs.