ISO ratings are calculated for each community by local fire departments, so the exact frequency will vary depending on where you live. However, ISO ratings are typically calculated at least every four to five years, and PPC rating will be updated based on the findings. Following the evaluations, fire departments can make upgrades to their training and systems to help increase their scores during the next rating period. If a fire department closes or experiences changes in its staffing or water supply, or makes other types of changes, the community might receive a lower ISO rating during the next evaluation.