I am in the business of the detection of toxic gases, so to me it is the smell of money.
Heat has some to do with it, however, the openness of the sewer systems and often the combination of sewers and storm drains tend to be the culprit. The gas you smell is Hydrogen Sulfide (H2S) and is water soluble so aeration can have a "short term' effect.
About safety: Hydrogen Sulfide has a very low odor threshold; for most people in the order of 0.01PPM (parts per million). H2S is considered toxic if the exposure is in excess of 10 PPM continuously (avg) over an 8-hour period or an instantaneous exposure of 20+ PPM. In other words, in open air there is no way you can be harmed from the gases that escapes the sewer.