Heat Impacts on Vulnerable Populations. Pregnant: Extreme heat events have been associated with adverse birth outcomes such as low birth weight, preterm birth, infant mortality, and congenital cataracts. Newborns: Newborns are extra sensitive to heat because their ability to regulate body temperature is limited. Children: Young children and infants are particularly vulnerable to heat, as their bodies are less able to adapt to heat than adults. Those under four are especially vulnerable. Elderly: Older adults, especially those who have preexisting diseases, take certain medications, live alone or have limited mobility are at higher risk for heat illness. Chronic Illness: People with chronic medical conditions are more likely to have a serious health problem during a heat wave.