Walking and cycling impact health in three ways: accident outcomes, polluted air inhalation, and exercise benefits. Actual impacts have been studied, but the decision to practice these modes depends on users' perception of health benefits and risks. As benefits outrun risks, for the promotion of active transportation, it is important to make users perceive the actual positive balance. This paper is a literature review on accident risks, polluted air risks, and exercise health benefits of active modes, both actual impacts and perceived impacts. The analysis details the factors affecting those actual impacts or perceived impacts. Individual factors include demographical (age, gender), socio-economical, experience, and psychological factors changing the actual risk or benefit level, and attitude (image, fear) towards risks and benefits. Environment factors include natural environment (weather, season, luminosity), and built environment (traffic volume and speed, road facilities, both mid-block and at intersections). Over 200 scientific papers were reviewed. The paper summarizes the main findings of the literature, but also point out the missing elements in existing research, and the discrepancy between risks and benefits on the one hand, and between actual impact and perceived impact on the other hand. These results will be used to design a survey to improve the knowledge of users' perception of active modes benefits and risks.