Once a rule is adopted, it must be implemented (i.e. enforced). The implementation process occurs typically in two stages. Stage one involves the notification of the impacted industries that they are subject to a new regulation. The timeframe for notification varies considerably depending upon the nature of the rule. The installation of some control equipment may require considerable source modification and design lead times. Also in many locations, a source cannot begin any modifications that can impact emissions (positively or negatively) without a permit to construct. Thus, the source may be allowed several years before compliance is required. In this case, the source is notified of the new regulation more than a year in advance of the compliance date. It should be noted, however, that few agencies consider lack of knowledge of a rule to be a reason for non-compliance. It is also common that sources be notified of upcoming regulatory requirements during routine source inspections.

If a permit is required, construction at a source is not allowed until the construction permit is issued. This can take months to years in some cases and must be built into the compliance timeline. A permit to construct typically involves an inspection by the regulatory agency prior to construction initiation followed by inspections during and after construction. The size of the source typically dictates the number of actual inspections that are taken by a regulatory agency. A typical cut-point for determining the level of review is 100 tons/year of emissions of any regulated pollutant. Sources emitting larger than 100 tons/year of a regulated pollutant are termed major sources in the United States and can receive multiple inspections during construction. Smaller sources may receive only one inspection following compliance with a new control requirement.

The modification of a major source that impacts its emissions (either an increase or decrease) will almost always result in the requirement of a source test at the cost of the source. The regulatory agency usually wants to observe this testing as well. The only exception to this is in the case of fugitive emissions where there may be no feasible technology for making emission measurements.

Sources are routinely inspected in order to ensure that they stay in compliance with relevant regulations. The number of inspections that are applied to a source depends upon the size of the source and the available resources for the inspecting agency. Since it is typically impossible to inspect sources at the ultimate rate, compliance agencies normally develop a program of prioritization for inspections. Compliance agencies normally do not make public their inspection criteria for fear of allowing sources to better anticipate the number and time of inspections that they might receive. However, it is valuable to develop and publish general criteria that will establish inspection priorities.

Criteria used to determine how often inspections occur are as follows:

1. Potential amount of emissions that could be released. The potential emissions to be released are different from the actual emissions that are being released. For example, a controlled source might, on paper, emit less than an uncontrolled source. However, mismanagement of the controlled source could actually produce more emissions than mismanagement of the uncontrolled source. Thus, the controlled source has a higher potential to emit.

2. Toxicity of emissions being released

3. Violation history

4. Complaints from the public

5. Potential to pollute (i.e. a natural gas boiler with no controls has little potential to increase emissions or violate even in the cases when emissions are large or the violator has a history of violations)

There are normally two types of inspections that are carried out. One type is the scheduled inspection and the other is the surprise inspection. In most cases, it is not feasible or reasonable for a source to alter its emissions because of the basic design of the operation. Thus, there is no concern that this type of source might operate in one manner when it is being observed and in another manner when it is not being observed. In this case inspections are set up in advance with the source to be inspected. It is worth noting that much of the public believes that sources increase emissions at night or on weekends and want agencies to make surprise inspections in any case. In cases where sources might have a reason to increase emissions when inspections are not anticipated or when it is difficult to observe emissions, such as at night, then surprise inspections can be the selected approach. Compliance agencies in the U.S. and elsewhere have the authority to get a Search Warrant and force entry into a facility at any time they feel there is reason to believe that emissions are increased beyond requirements.