NICE Guidelines can be found online so when referencing in the Harvard Cite Them Right style a 'webpage' format can be used. For example, a full reference will look like:
National Institute for Health and Care Excellence (NICE) (2017) Air pollution: outdoor air quality and health. NG70. Available at: https://www.nice.org.uk/guidance/ng70 (Accessed: day month year).
Use the date the guideline was last updated.
The first in text citation will be:
National Institute for Health and Care Excellence (NICE) (2017) or (National Institute of Health and Care Excellence (NICE), 2017)
Any subsequent citations will be;
NICE (2017) or (NICE, 2017)
