2. Member States shall establish and make available through a public source, in an easily understandable manner, an air quality index covering hourly updates on at least sulphur dioxide, nitrogen dioxide, particulate matter (PM10 and PM2,5) and ozone, provided that there is an obligation to monitor those pollutants pursuant to this Directive. That index may include additional pollutants, when relevant. Insofar as possible, the air quality index shall be comparable across all Member States and follow WHO recommendations. The air quality index shall build on the air quality indices at European scale provided by the European Environment Agency and include information regarding impacts on health, including information tailored to sensitive population and vulnerable groups. Alternatively, Member States may use the air quality index provided by the European Environment Agency to meet the requirements set out in this paragraph. If a Member State decides not to use the index provided by the European Environment Agency, a reference to that index shall be made available at national level.