Well pheromones are biological compounds, and as with anything biological, intense heat tends to destroy and denature their structure.
If Johnny's radiating enough heat by simply just being, and Poison Ivy has to stay a distance away and rely on wind currents to carry her pheromones to him, by the time the pheromones reach a certain radius of him, wouldn't the heat have already destroyed the chemicals before he could actually breathe them in?