State measures will be pre-empted, though, if they conflict or interfere with the Chemical Facility Anti-Terrorism Standards.

While this essentially reiterates language used in the advance notice of rulemaking issued by the US Department of Homeland Security (DHS) last December, the DHS has emphasised that the provisions finalised this month do not constitute a broad "field pre-emption" for high-risk facilities. Nor does the Department believe any existing state laws and regulations on chemical security are being applied in ways that would impede or interfere with the new federal rules.

That may come as little comfort to the state officials, environmental activists and their congressional supporters who wanted the pre-emption issue struck out of the new regulations and left to the discretion of the courts.

But the DHS' insistence on pre-emption authority where there is conflict between state and federal regulations will play better with the chemicals industry, which has backed pre-emption in the interests of a level playing field after already making its own substantial investments in security upgrades.

The pre-emption issue has been a particular moot point in the debate over new rules designed to guard against terrorist attacks on facilities storing potentially dangerous chemicals, particularly those close to high-density population centres.

Section 550 of the Department of Homeland Security Appropriations Act of 2007, signed into law last October, required the US Secretary of Homeland Security to promulgate interim final regulations "establishing risk-based performance standards for security of chemical facilities" by 4 April 2007. The interim final rule on Chemical Facility Anti-Terrorism Standards was announced on 2 April and published in the 9 April Federal Register.

The new rules take effect 60 days after the publication date, while covered facilities identified by the DHS as potentially high-risk will have 120 days to provide the necessary information for the risk-assessment procedure laid out in the regulations. The Department has also released, for a 30-day comment period, a proposed list (Appendix A) of "chemicals of interest" that may put facilities housing them in the high-risk category.

The interim final rule retains the basic framework of a risk-based, tiered approach to reducing the vulnerability of chemical facilities. Owners of facilities housing certain quantities of specified chemicals will have to carry out a preliminary assessment online to determine which risk category they fall into. If the facility qualifies as high-risk, the owners will need to prepare and file a security vulnerability assessment and site security plan, which will then be validated through audits and site inspections.

Security standards will be tied to specific outcomes, such as securing the perimeter of the facility and critical targets, controlling access, preventing internal sabotage and deterring theft of potentially dangerous chemicals. Non-compliance could lead to civil penalties of up to US$25,000 a day or facility closures.

The DHS' initial estimate, Homeland Security Secretary Michael Chertoff told a press conference, was that as many as 7,000 chemical facilities would fall into the four tiers of the high-risk category (the higher the tier, the tougher and more urgent the security measures required), with around 300-400 qualifying for the top two tiers.

On the pre-emption issue, Chertoff said: "So as we sit here now, we're not envisioning that this is going to be a problem … with respect to any of the existing regimes of chemical security in the states."

There are concerns, however, that moves in New Jersey to require the use of inherently safer technologies (ISTs) - an industry bugbear - as a second tranche of the state's own chemical security programme could founder, as they are still at the proposal stage and hence not eligible for 'grandfathering' under the new rules.

The clarifying language in the interim final rule states: "This regulation is not intended to be the equivalent of field pre-emption for facilities determined to be high risk. Instead, it is only meant to indicate that the regulation is not to be conflicted with, interfered with, hindered by or frustrated by state measures under long-standing legal principles."

As Chertoff pointed out, any grey areas would ultimately be resolved by the courts anyway. "I don't think that the mere fact that a state adds some additional strength [to their regulations] necessarily creates a pre-emption issue," he commented.

"Now, obviously there are different ways to do things, and I can't tell you that a state that takes measures that actually conflict with what we're doing wouldn't have a pre-emption problem."