It must be noted that local burn conditions could significantly change these relative levels. Based on very limited data, PCDD/F and other persistent bioaccumulative toxic (PBT) emissions varied greatly from source to source and exhibited significant variations within source categories. This high degree of variation is likely due to a combination of factors, including fuel composition, fuel heating value, bulk density, oxygen transport, and combustion conditions. This highlights the importance of having acceptable test data for PCDD/F and PBT emissions from open burning so that contributions of sources to the overall PCDD/F and PBT emissions inventory can be better quantified