Which of the following conditions is necessary for effective fumigation?

Effective fumigation requires the structure to be an enclosed environment. This is critical because fumigants need to be contained within the space to work effectively. Proper sealing of the structure prevents the escape of the gases and ensures that the pests inside come into contact with the fumigant throughout the treatment period.

If the structure is not enclosed, the fumigant will dissipate quickly, reducing its efficacy and potentially allowing pests to survive the treatment. Additionally, an enclosed space helps to maintain the necessary concentration of the fumigant gas over time, which is essential for eliminating termites and other pests.

The other conditions mentioned do not support effective fumigation. Open air access would allow the fumigant to escape, moisture levels above 50% can negatively impact fumigation efficacy, and the presence of natural light is not relevant to the process at all. Therefore, an enclosed structure is fundamental for successful fumigation practices in pest control.