The World Health Organization (WHO) invites vendors to submit an Expression of Interest (EOI) for a Long Term Agreement for maintenance, improvement, and further development of the WHO air quality and climate-related health impact assessment tools: AirQ+, CLIMAQ-H, and iSThAT which address the following objectives: Maintenance of AirQ+ and CLIMAQ-H, including debugging, updating XML meta-files, ensuring coherence of file structures, and UI alignment. Improvements to help repository, export functionalities, life table calculations, economic assessment module, RR calculations including carcinogenic pollutants, and enhanced household air pollution interface. Complete redesign of iSThAT as a standalone cross-platform tool with a new GUI aligned with AirQ+ and CLIMAQ-H. Evaluation I and II with experts and users, with refinement based on results. Delivery of full documentation and source code for all tools. AirQ+, CLIMAQ‑H, and iSThAT are WHO software tools used to quantify the health impacts associated with air pollution, climate‑related stressors, and urban transport emissions. AirQ+ and CLIMAQ‑H are numerical applications that calculate health and economic effects of air quality and climate mitigation scenarios, while iSThAT supports assessment of health and carbon impacts from mobility‑related interventions. WHO is seeking support to maintain, update and further develop these tools to ensure accuracy, reliability and usability for Member States, technical experts and practitioners. WHO intends to invite selected vendors to participate in a formal solicitation, via a Request for Proposals (RFP), at a later stage, for the above requirements.  Complete details of the requirements will be included in the solicitation documents.