Air permitting challenges for portable machinery
Portable generator engines, rock crushers, and aggregate processing units are designed to move from site to site. However, under certain conditions, these mobile units may be reclassified as stationary sources of air pollution. This shift in classification can trigger regulatory requirements that operators may not anticipate, including permitting, emissions monitoring, and reporting obligations.
Defining stationary sources under the Clean Air Act
The Clean Air Act defines a stationary source as any building, structure, facility, or installation that emits (or has the potential to emit) air pollutants. The Environmental Protection Agency (EPA) further clarifies that portable equipment becomes stationary if it remains at a single location for more than 12 consecutive months. This rule applies regardless of whether the equipment was originally designed to be mobile.
For example, a portable diesel engine used to power a rock crusher may be considered stationary if it operates at the same site for over a year. Once reclassified, the equipment may be subject to federal standards such as New Source Performance Standards (NSPS) or National Emission Standards for Hazardous Air Pollutants (NESHAP).
State-level interpretations and variations
While EPA provides overarching guidance, individual states often implement their own rules and permitting frameworks. These can vary significantly depending on local air quality concerns, industrial activity, and enforcement priorities.
California’s Portable Equipment Registration Program allows certain engines and equipment to operate statewide without obtaining site-specific permits, provided they meet emission standards and are properly registered. In contrast, Texas requires a permit for rock crushers that operate at a site for more than 12 months, aligning closely with EPA’s definition of stationary sources. Maine uses a Crusher Identification Number system to track emissions from portable units and ensure compliance with state regulations.
Some states also impose thresholds based on horsepower, fuel type, or emission potential. Equipment that exceeds these thresholds may require a stationary source permit even if it's moved periodically.
Compliance challenges and enforcement risks
Misunderstanding the distinction between portable and stationary sources can lead to significant compliance issues. Operators may assume that mobility exempts equipment from permitting, only to discover that prolonged use at a single site has triggered regulatory oversight. Failure to obtain the proper permits or meet emission standards can result in fines, enforcement actions, and operational delays.
Recordkeeping is another common challenge. Regulators often require documentation showing how long equipment has been at a site, its emission characteristics, and any relocations. Without accurate records, operators may struggle to prove that their equipment qualifies as portable.
Best practices for operators
To be compliant, operators should:
- Monitor and record how long each unit remains at a location,
- Understand the emissions profile and horsepower of their equipment,
- Review both federal and state regulations before deploying portable units,
- Maintain detailed records of equipment movement and usage, and
- Consult with environmental agencies when planning long-term projects.
Proactive communication with regulators can help clarify requirements and avoid costly surprises. In some cases, applying for a general or portable permit may be the simplest way to ensure compliance.
Key to Remember: Portable equipment doesn’t stay exempt forever. If it remains at one site too long, it may be regulated as a stationary source, bringing new rules, responsibilities, and risks.




















































