Seveso Lower-Tier vs Upper-Tier: Differences and Obligations
In France, industrial sites handling hazardous substances above certain quantities fall under the Seveso regime, which transposes the European Seveso III Directive (2012/18/EU) into the nomenclature of installations classified for environmental protection (ICPE). Two classification tiers exist: lower-tier and upper-tier. They cover different product quantities and different obligations for the operator.
How are the lower-tier and upper-tier thresholds set?
A site's classification depends on the cumulative quantities of hazardous substances it may hold, compared against the thresholds in Annex I of the Seveso III Directive. Each category of substance (explosive, oxidising, toxic, hazardous to the environment, and so on) has its own lower-tier and upper-tier threshold values, expressed in tonnes. When several hazardous substances coexist on the same site, an aggregation rule applies: the sum of the ratios between the quantity held and the reference threshold for each substance determines whether the site crosses a tier, even if no single substance reaches it on its own.
A site can be classified as Seveso upper-tier without holding an individually higher-than-threshold quantity of any single substance: it is the combination of several hazardous products that triggers the classification.
Comparing the regulatory obligations
Lower-tier vs upper-tier: the regulatory obligations
This table summarises the main obligations specific to each Seveso classification tier.
| Lower-tier | Upper-tier | |
|---|---|---|
| Hazard study | Required, proportionate to the risks | Required, in-depth with worst-case scenarios |
| Internal emergency plan (POI) | Required | Required |
| External emergency plan (PPI) | Not required | Required, coordinated by the prefect |
| Technological risk prevention plan (PPRT) | Not required | Required |
| Safety management system (SGS) | Recommended, not formalised | Required and documented |
| Site monitoring committee (CSS) | Not required | Required |
| ICPE inspection frequency | Every 3 years | Annual |
| Public-utility easements around the site | No | Yes |
Why the hazard study matters
Whether a site is lower-tier or upper-tier, the hazard study remains the foundation of the prevention approach. It identifies major-accident scenarios, assesses their likelihood and severity, and determines the risk-control measures to implement. For an upper-tier site, this study must demonstrate stronger risk control and include worst-case scenarios, consistent with the requirements of the PPRT and the PPI.
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Anticipating a change of tier
Seveso classification is not fixed. A change in the process, additional storage, or the introduction of a new hazardous substance can push a site from lower-tier to upper-tier. It is advisable to reassess the classification whenever stored quantities change significantly, before commissioning, in order to anticipate the administrative steps (updating the hazard study, drawing up the PPI, informing the CSS) that can take several months.
