NOM-085-SEMARNAT: fixed-source emission limits and how to comply

NOM-085-SEMARNAT-2011 is the Mexican Official Standard that sets the maximum permissible atmospheric emission levels for indirect-heating combustion equipment, such as boilers, steam generators, thermal-fluid heaters, furnaces and dryers. Its purpose is to protect air quality by regulating what these units release into the atmosphere when they burn conventional fuels.

Specifically, the standard sets limits for smoke, particulate matter, carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen oxides (NOx). If your plant operates boilers or any indirect-heating combustion equipment, this standard very likely applies to you.

What NOM-085-SEMARNAT-2011 regulates

The standard is part of the catalogue of Mexican Official Standards issued by SEMARNAT. Its scope covers fixed sources (industrial, commercial and service facilities) that use indirect-heating combustion equipment, that is, units in which the combustion gases do not directly contact the product but heat an intermediate fluid such as water, steam or thermal oil.

The standard was published in the Official Gazette (DOF) on 2 February 2012, came into force sixty days later and replaced NOM-085-SEMARNAT-1994. It was amended by an agreement published on 15 December 2014 that adjusted the compliance calendars for sulfur dioxide. It is mandatory for both federal and local jurisdiction fixed sources in industry, commerce and services.

The threshold lies in the equipment's nominal thermal capacity: the standard applies from 530 megajoules per hour (MJ/h), roughly 15 boiler horsepower. Excluded are units below that capacity, domestic heating and water heating equipment, gas turbines, auxiliary and standby equipment, and cases where bioenergy fuels are used. For gas turbines used in power generation, a transitory provision sets a NOx limit of 70 ppmV for units above 106 GJ/h until a specific NOM exists.

Critical zones

The standard is stricter in the so-called critical zones, defined as areas where topography and weather hinder dispersion or where high pollutant concentrations are recorded. They are the Guadalajara Metropolitan Zone, the Monterrey Metropolitan Zone, the Valley of Mexico Zone, the Coatzacoalcos-Minatitlán, Irapuato-Celaya-Salamanca, Tula-Vito-Apasco and Tampico-Madero-Altamira industrial corridors, the municipality of Ciudad Juárez and the Tijuana and Rosarito area.

Who it applies to

It binds the individuals and legal entities responsible for fixed sources operating indirect-heating combustion equipment above the capacity threshold. In practice this covers a wide range of industries with boilers and thermal processes: food, textile, chemical, paper, metallurgy and many more. The key is not the sector but the type and capacity of the equipment.

Maximum permissible levels: how they are structured

NOM-085's limits are not a single value; they depend on three combined factors: the equipment's thermal capacity, the fuel type (gaseous, liquid or solid) and the location of the source, with stricter requirements in so-called critical zones. The standard organises these values in tables that distinguish existing equipment from new equipment, with stricter requirements for the latter.

Because the specific values vary case by case, the reference must always be the standard's text in force published in the Official Gazette (DOF) and its amendment agreements. What matters for the plant is knowing which table and which fuel-and-capacity combination its equipment falls under. Some application rules worth knowing:

• Reference conditions: all results are corrected to 25 °C, 1 atmosphere and 5% oxygen on a dry basis.
• Several fuels at once: the emission is compared with the limit of the most permissive fuel in use; for example, a gas and liquid mix is assessed against the liquid fuel limit.
• Several units at one source: each unit can be assessed separately, or emissions and limits can be weighted across all units by thermal capacity. In expansions, new units must meet the new equipment table before weighting.
• Events in which the limit may be exceeded: start-ups, soot blowing, burner maladjustment, or shutdown and maintenance of control equipment. They must be logged with their duration, and together cannot exceed the hours equivalent to 36 calendar days of operation in a calendar year.
• Sulfur dioxide in critical zones: the fifth transitory article, amended in 2014, required existing units above 530 GJ/h located in critical zones to meet a 600 ppmV SO2 limit by 1 January 2019 at the latest, with gradual compliance calendars and annual notices to SEMARNAT.

How to comply: measurement, monitoring and reporting

Complying with NOM-085 means demonstrating, with data, that emissions stay below the limits. The process rests on three pillars:

1. Measurement: sampling stack gases and particulates using the reference methods the standard specifies (isokinetic sampling for particulates, and instrumental methods for CO, NOx and SO2), through an accredited and approved laboratory. Instrumental analyses require sampling for at least one hour with a minimum of 60 readings at equal intervals, with the unit in normal operating conditions. For units below 1,000 GJ/h, SO2 can be determined by stack analysis or through emission factors or a mass balance based on the sulfur content of the fuel.
2. Monitoring: keeping periodic track of equipment performance and its emissions, at the frequency corresponding to the capacity. Sources installing units above 1,000 GJ/h for the first time with fuels above 1% sulfur must have a continuous emissions monitoring system for SO2, opacity and oxygen, with at least 75% validated data during operation. If control equipment is used to comply, it must operate at least 90% of the combustion equipment's annual operating time.
3. Reporting and logbook: retaining the results and reporting them to the authority, both in licensing procedures and in the annual emissions declaration. The standard also requires an operation and maintenance log, printed or electronic, with the name, brand and capacity of each unit, the control and measurement equipment, and daily records of date, shift, fuel consumption and type, percentage of design capacity at which the unit operated and average stack gas temperature.

Good burner maintenance and correct combustion tuning not only help meet the standard, they also reduce fuel consumption and with it the facility's carbon footprint.

Conformity assessment and penalties

Compliance can be certified before PROFEPA or an accredited and approved verification unit, which review the logbook, the laboratory reports and the calculation records, check the continuous monitoring system where applicable and issue an opinion on the degree of compliance. Enforcement lies with PROFEPA and with state and municipal governments within their remit, which can carry out verification visits regardless of that procedure. Non-compliance is penalised under the LGEEPA, whose article 171 provides for fines of 30 to 50,000 days of minimum wage (a reference now calculated in UMA), closure and suspension or revocation of licences, and under the atmospheric regulation, with fines of 20 to 20,000 days.

It is worth distinguishing two planes that are often confused. NOM-085 regulates the concentration of local pollutants (particulates, SO2, NOx, CO) leaving the stack, in order to protect air quality. Calculating greenhouse gases, by contrast, measures the contribution to climate change (mainly CO2) and relies on emission factors applied to fuel consumption. They are separate obligations, but they share the same data source: how much and what your plant burns.

Relationship with the LAU, the COA and the RETC

NOM-085 does not work in isolation; it is embedded in the federal atmospheric-management system:

• For federal-jurisdiction fixed sources, the Single Environmental Licence (LAU) is the operating authorisation, which reflects the combustion equipment and its emission conditions under NOM-085.
• The Annual Operating Certificate (COA) is the annual report through which emissions are declared to SEMARNAT, and it is based on the measurements the standard requires.
• The COA's information feeds the Pollutant Release and Transfer Register (RETC), the country's public emissions database.

NOM-085 also coexists with other environmental-assessment instruments, such as the Environmental Impact Statement (MIA) for new projects, all underpinned by the LGEEPA. For an overview of the Mexican framework, see our guide on environmental laws, NOMs and certifications in Mexico.

Frequently asked questions

Which equipment does NOM-085-SEMARNAT-2011 apply to?

Indirect-heating combustion equipment (boilers, steam generators, thermal-fluid heaters, furnaces and dryers) with a nominal thermal capacity of 530 MJ/h or more that burns conventional fuels. Smaller units, domestic equipment, gas turbines, auxiliary and standby equipment and units running on bioenergy fuels fall outside its scope.

Which pollutants does it limit?

Smoke, particulates, carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen oxides (NOx), with values that depend on the fuel, the equipment capacity and the source location.

How is compliance demonstrated?

Through stack measurements using reference methods performed by an accredited laboratory, periodic monitoring, and reporting to the authority, in particular via the Annual Operating Certificate.

If you want to centralise your equipment's combustion data and keep it traceable for environmental reporting, you can calculate your plant's carbon footprint with Manglai.