The impact of ozone exposure on trees, plants and ecosystems often is assessed using a seasonal index known as a "W126 index." A W126 index, named after portions of the equation used to calculate it, is a weighted index designed to reflect the cumulative exposures that can damage plants and trees during the consecutive three months in the growing season when daytime ozone concentrations are the highest and plant growth is most likely to be affected.

Here is a list of resources for calculating W126 index values.

• How the W126 index value is calculated (PDF)
• W126 daily index calculation in Excel (XLS)(1 pg, 35 K)
• W126 3-year average calculation in Excel (XLS)(1 pg, 782 K)

Also available below is a spreadsheet showing the W126 index values derived from ozone monitoring data from 2001 to 2013.

• W126 index values and values of design value metric for current national ambient air quality standards for ozone at monitoring sites with adequate data (2001-2013)(1 pg, 878 K)
• How the above calculations were performed (PDF)