Visits to the emergency department (ED) are stressful for any patient and pose special challenges and potential barriers to care for those with autism spectrum disorder (ASD). In addition, people (both children and adults) with ASD differ in their perception of, or responses to, environmental stimuli. Some people with ASD present with sensory defensiveness that is environmentally moderated, but there is a gap in our knowledge about patterns or thresholds of stimulation that kindle behavioral responses. We systematically recorded visual and auditory stimuli in an ED. Data were collected in four areas, hallway, doorway of waiting room, chair in waiting room, and treatment room, over several weeks and times of day. The mean light intensity ranged from 127 to 184 lux. There were significant differences (F = 16.77, df = 3, p > 0.001) between the five ED locations. Mean sound levels ranged from a high of 63.291 dB in the waiting room chair to a low of 51.289 dB in the treatment room. The highest consistent level of noise was recorded while sitting in the waiting room chair (60.657–63.291 dB). The findings suggest significant differences in light and sound intensity in common ED locations, with the highest mean readings of light and noise in the waiting areas. The differences and descriptions of light and sound patterns support the need for additional studies in both the measurement of the ED environment and its role in stimulating sensory defensiveness in people with ASD.