To aid interpretation, the average number of grass pollen grains per cubic meter of air is classified into LOW, MODERATE, HIGH and EXTREME. For grass pollen, the classification is as shown in the following table:

Number of grass pollen grains (Classification)

• 0 to 19 (LOW)
• 20 to 49 (MODERATE)
• 50 to 99 (HIGH)
• ≥ 100 (EXTREME)

We expect most grass-sensitive people will experience symptoms on days with HIGH grass pollen levels. For other pollen types and for mould spores, we don’t yet know the levels that cause symptoms and have simply divided the expected range of daily concentrations up. In future, we will use survey scores submitted through the app to help us identify where we should draw the HIGH cut-off for each pollen type. That’s one reason why every survey score you submit helps us better understand what’s causing hay fever.