Update README.md

README.md

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 2. Recycle when unusable
 3. Degrade/incinerate unless endlessly recyclable (i.e. fibers don't shorten)
 
+Key challenges (need to extract challenges from WHO document)
+
 Tech Interventions for reusability
 - Sorting tech: Evaluation of quality of PPE (e.g. light sensor for opacity, whether its scratched up)
 - Reusable Materials (e.g. face shield that can be autoclaved)