Implications of air-conditioning use on thermal perception in open spaces: A field study in downtown Rio de Janeiro

• We discuss the consequences of AC-usage in climate-responsive urban design for Rio de Janeiro.
• Field data showed differences among respondents who reported to have or not access to AC in terms of thermal sensation votes.
• Impact of a prolonged AC-usage on the thermal perception of passers-by in outdoor spaces.
• Using Rio de Janeiro's TMY2 data, a significant rise in the annual percentage of heat stress hours was found for AC-users.
• Results point to the need for assessing usage/ownership of air-conditioning equipment in outdoor comfort surveys.

The interference of thermal history in a person's thermal perception has been explored by several authors and this has been one of the pillars of the concept of adaptive comfort. Given that a general trend towards a more frequent usage and ownership of air-conditioning (AC) units or central systems is taking place worldwide, and considering the fact that AC-users show a diminished tolerance to heat stress when compared to more acclimatized persons, this paper discusses the consequences of AC-usage in climate-responsive urban design. Rio de Janeiro (22° 54′ 10″S, 43° 12′ 27″W) has a tropical climate with summer rains (Aw) and is characterized by frequent heat stress conditions. In such context, air conditioning is a common strategy to reduce indoor heat in buildings. The purpose of this study was to analyze the influence of AC-usage in the thermal perception of a large group of urbanites as regards to Rio's outdoor thermal conditions. Thermal votes were assessed by means of standard questionnaires in several field monitoring campaigns. Meteorological variables, measured on site with a portable weather station were post-processed to UTCI values (Universal Thermal Climate Index). Field data showed significant differences among groups of respondents who reported having access to AC-units (or central systems) at home and/or at work in terms of thermal sensation votes. By applying the derived set point UTCI values for two subgroups (AC-users against non users) to Rio de Janeiro's TMY2 (typical meteorological year) data, a significant rise in the annual percentage of heat stress hours was found for AC-users, in magnitude even higher than expected changes in annual heat stress percentages due to climate-responsive urban planning and thus increasing the demands on heat stress mitigation strategies.