Under high humidity, the volatility of a pesticide tends to

• A. decrease
• B. increase
• C. not affect
• D. vary with wind speed

Answer: (A)

When humidity is high, the air already contains a lot of water vapor, which reduces the vapor-gradient that drives evaporation from a pesticide surface. With less driving force for the pesticide molecules to diffuse into the air, fewer molecules escape per unit time, so volatilization decreases. Temperature and air movement also play roles, but the moisture in the air itself tends to hinder volatilization, not enhance it. So, under high humidity, volatility tends to decrease. The idea that it increases or that humidity has no effect doesn’t fit, and wind can influence volatilization, but humidity specifically acts to slow it down.