The NPI program provides a significant amount of information about emissions to the environment. Users should take the following factors into consideration when reviewing the data:
- The NPI contains two types of data — emissions from individual facilities and diffuse emissions. If facilities trip one of the NPI reporting thresholds, facilities must calculate their emissions and provide this data annually for the NPI. Diffuse data shows the contribution of non-industrial sources and selected sub-threshold industry to Australia's emissions. The diffuse data is not collected annually and is not necessarily for the particular reporting year being examined. For example, diffuse data may be from a study completed in 1998—99; however it is the most up-to-date information available at that time.
- It is important to note that NPI substances range in toxicity. For example, a small amount of a highly toxic substance may be more significant than a larger emission of a less toxic substance.
- The various NPI substances have distinct properties and levels of toxicity and it is therefore meaningless to add together emissions of different substances. This does not provide a measure of total pollution.
- Some substances of concern are not reported to the NPI. For example, the Australian Government regulates pesticides and tracks emissions of greenhouse gases through programs other than the NPI.
- The NPI is a database of emissions which have been calculated using a variety of techniques. The accuracy of these calculations may vary according to the technique used.
- Changes in reported emissions can be due to a variety of factors, including improved environmental performance, increases and decreases in production, changes to the process, installation of pollution-control equipment such as fabric filters, first time reporting of a substance for the first time and updated emission calculation techniques.
- NPI data shows what is being released from an emission source, but whether emissions of a substance cause pollution to air, land or water depends on additional factors — such as the amount of substance emitted, where it is emitted, the weather conditions at the time, and the topography of the land.
- All emission amounts reported have been rounded to two significant figures — totals may differ from the sum of the individual amounts on these reports because of this rounding.
For more information read the Interpretive guide for the NPI