Which of the following conditions are most favorable for volatilization to occur?

• A. Cool air temperature, low relative humidity
• B. Cool air temperature, high relative humidity
• C. Warm air temperature, low relative humidity
• D. Warm air temperature, high relative humidity

Answer: (C)

Volatilization depends on two main things: how readily the pesticide can turn into vapor (vapor pressure) and how easily that vapor can move into and be carried away by the surrounding air. Warmer temperatures raise vapor pressure, making it easier for the chemical to become a gas. Low relative humidity means the air isn’t already full of water vapor, so it can absorb more pesticide vapor, allowing it to diffuse away more readily. Put together, warm air and dry air create the conditions most conducive to volatilization.

That’s why warm air temperature with low relative humidity is the best fit among the options. Cooler temperatures lower vapor pressure and reduce volatilization; high humidity makes the air already relatively saturated with water vapor, which slows the uptake of pesticide vapor into the air. So, even if the temperature is warm, high humidity doesn’t promote volatilization as effectively as when humidity is low.