Heat rashes appear when sweat gets trapped under the skin. Because babies have smaller sweat glands and are less able to regulate their body temperature, they are more vulnerable to heat rash than adults.
Tight clothes, swaddles, and blankets can also cause heat rashes. In most cases, the rash will go away without treatment.
Babies are more likely to get heat rash for several reasons:
They have little control over their environment and cannot take off extra clothing or move away from heat sources.
Their bodies are less effective at regulating temperature.
They tend to have more skin folds, which can trap heat and sweat.