This document outlines a Harmonized Heat Warning and Information System (HWIS) for dealing with heat events. The aspects of this HWIS include:

• Governance: roles, responsibilities, and decision-making structures
• Triggers for the issuing of various heat warning to the public health units (PHUs) from a central source, and mechanisms for disseminating these warnings
• Definitions of warnings, along with suggested core activities
• Communications messaging to support the various warning levels
• Planning guidance to support PHUs and local partners in their heat response planning

This document is intended to support planning for boards of health and PHUs in Ontario. Other levels of government and potential partners may also choose to use the guidance contained in this document.