From an epidemiological perspective, there is no definition of pollution spikes in terms of health effects. At the pollution levels observed in Western countries, there are no thresholds below which no health effects would be observed in the population, whether for long-term or short-term exposure. 

Despite their small contribution to overall health impacts, pollution spikes provide an opportunity to raise awareness among stakeholders and the general public about the health effects of air pollution.

Air pollution spikes are identified by exceedances of thresholds (daily or hourly) set by French regulations.

Only a retrospective analysis of a spike allows the effects attributable to pollution to be identified. It is indeed very difficult, if not impossible, to directly measure its impact on healthcare utilization.
The contribution of pollution spikes to the health impacts of pollution varies depending on the cities and years considered. It can account for up to 20% of the overall impact of short-term pollution in certain cities, but it remains well below the effect of long-term exposure.
The most recent major pollution spike in France occurred in December 2016. The highest concentrations were observed in the Normandy, Hauts-de-France, Île-de-France, Bourgogne-Franche-Comté, and Auvergne-Rhône-Alpes regions.