This is the first study to evaluate the indoor air quality of markets using the “Indoor Environmental Index”. In the study, carbon dioxide (CO2), relative humidity, temperature, particulate matter, and total volatile organic compounds were measured as indoor air quality parameters in four different markets in Istanbul during the COVID-19 pandemic. Data were analyzed and evaluated using IBM SPSS Statistics 22 program. While CO2, PM2.5, PM10, humidity, and temperature had a statistically significant difference in different markets, no statistically significant difference was found for NO2 and total volatile organic compounds (p > 0.05). Considering the different hours in a day, it was determined that there was a statistically significant difference for all parameters. The highest and strongest correlation between the parameters was found between PM2.5 and PM10 (r = 0.703, p < 0.01). The IEI values for 4 different markets in different time intervals in a day were found as 6.862, 6.775, 8.816, and 6.244, respectively. The highest and lowest Indoor Environmental Index values were calculated in market2 (7,525) and market4 (4,936), respectively. Indoor air quality parameters had an impact on the IEI results as they affected the pollution index and the discomfort index. As a result of the study, it was seen that the density of customers and products, the size of the closed area of the markets, and the capacity of ventilation equipment affect the indoor air quality. All these results were evaluated and suggestions were made about the visit times to the markets.