Studies reveal that ventilation drifts upward during constant-load submaximal exercise performed in a hot and humid environment. this is likely due to

The answer would be; a significant increase in blood temperature.

The muscle contraction from exercise will produce heat that will increase the body temperature. In a hot and humid condition, this will make the body temperature hard to be lowered. The increase of blood temperature will influence the respiratory control center, makes the respiration drift upward.