Small engine emissions statistics span a large California equipment population, historic national estimates, and modeled emission factors for different engine classes. The figures below identify their population, period, geography, unit, and limitations so that current California inventory estimates are not confused with historical uncontrolled-engine estimates or laboratory-style model factors.
Key Small Engine Emissions Statistics
The most broadly useful figures are:
- In 2021, California had an estimated 15.4 million small off-road engines in the California Air Resources Board (CARB) population context.
- In 2021, California had an estimated 14.0 million light-duty passenger cars in the comparison population.
- In 2021, residential lawn-and-garden equipment made up 61% of California’s SORE population.
- In 2021, commercial lawn-and-garden equipment made up 8% of California’s SORE population.
- In 2021, residential lawn-and-garden equipment represented 33% of California SORE emissions in CARB’s displayed mix.
- In 2021, commercial lawn-and-garden equipment represented 20% of California SORE emissions in CARB’s displayed mix.
- In 2021, commercial lawn-and-garden equipment represented 20% of summer smog-forming SORE emissions in California.
- For a 1972 estimate based on 1970 inventory inputs, U.S. lawn-and-garden four-stroke engines were estimated at 73,000 tons per year of exhaust hydrocarbons.
- For the same historic basis, U.S. lawn-and-garden two-stroke engines were estimated at 46,600 tons per year of exhaust hydrocarbons.
- For the report’s historic small-engine categories, total exhaust hydrocarbons were estimated at 128,000 tons per year in the United States.
- For the same historic basis, lawn-and-garden four-stroke engines were estimated at 878,000 tons per year of total carbon monoxide.
- For the same historic basis, lawn-and-garden two-stroke engines were estimated at 106,000 tons per year of total carbon monoxide.
- In EPA’s NONROAD2008a factor documentation, Class I baseline two-stroke HC was 207.92 g/hp-hr for nonhandheld engines under 225 cc.
- In the same documentation, Class I Phase 3 overhead-valve HC was 3.80 g/hp-hr for nonhandheld engines under 225 cc.
- In EPA’s Class IV handheld factors, baseline two-stroke CO was 718.87 g/hp-hr for engines over 20 cc and under 50 cc.
- In the same Class IV group, Phase 2 catalyst CO was 141.69 g/hp-hr.
- In the historic seasonal model, the central urban/suburban cell received 24.65% of annual emissions in June-August.
- In the historic report’s rounded regional summary, the central United States accounted for about 52% of the modeled national total.
Contents
- California equipment scale and source mix
- Pollutant and lawn-and-garden emissions data
- Engine technology and regulatory-phase trends
- Handheld and nonhandheld engine classes
- Seasonal and regional emissions patterns
California Small Engine Emissions Statistics: Equipment Scale and Source Mix
CARB’s 2021 SORE fact sheet places California’s small off-road engine population at 15.4 million engines, compared with 14.0 million light-duty passenger cars. SORE means spark-ignition engines rated at or below 19 kW, and the car figure is a comparison population rather than an emissions-equivalence claim. CARB, “SORE – Small Engine Fact Sheet”
The population mix and emissions mix use different denominators. That distinction matters because a category can have a smaller equipment share but a larger share of the displayed emissions mix.
| California SORE category | 2021 population share | 2021 SORE emissions share |
|---|---|---|
| Residential lawn and garden | 61% | 33% |
| Commercial lawn and garden | 8% | 20% |
| Federally regulated construction and farming | 11% | Not stated in the supplied emissions mix |
| Other equipment, including generators and pressure washers | 20% | Not stated in the supplied emissions mix |
CARB’s fact sheet reports residential lawn-and-garden equipment at 61% of the California SORE population and 33% of SORE emissions. Commercial lawn-and-garden equipment accounts for 8% of the population and 20% of the displayed SORE emissions mix, while federally regulated construction and farming equipment accounts for 11% of the population and other equipment accounts for 20%.
For summer smog-forming emissions, CARB reports commercial lawn-and-garden equipment at 20% of SORE emissions, despite its 8% population share. This comparison is specific to summer smog-forming emissions and should not be generalized to every pollutant or season. CARB, “SORE – Small Engine Fact Sheet”
Small Engine Emissions Data by Pollutant and Application
EPA’s historic report estimated uncontrolled small-engine emissions using 1970 inventory inputs for a 1972 estimate. These are not a current U.S. inventory; they describe the report’s historical categories and should not be presented as today’s national total. EPA, “Exhaust Emissions From Uncontrolled Vehicles and Related Equipment using Internal Combustion Engines”
| Pollutant or measure | Lawn-and-garden four-stroke | Lawn-and-garden two-stroke |
|---|---|---|
| Exhaust hydrocarbons, tons/year | 73,000 | 46,600 |
| Evaporative hydrocarbons, tons/year | 5,170 | Not stated |
| Total hydrocarbons, tons/year | 78,100 | 47,000 |
| Total carbon monoxide, tons/year | 878,000 | 106,000 |
| NOx as NO2, tons/year | 9,970 | 344 |
| Particulate, tons/year | 1,400 | 1,500 |
| SOx as SO2, tons/year | 1,200 | Not stated |
The same historic report estimated 128,000 tons per year of exhaust hydrocarbons for its full small-engine categories, which include miscellaneous four-stroke equipment as well as lawn-and-garden equipment. Its lawn-and-garden totals separately show 78,100 tons per year of total hydrocarbons for four-stroke engines and 47,000 tons per year for two-stroke engines.
Carbon monoxide was the largest quantified pollutant in the lawn-and-garden application table: 878,000 tons per year for four-stroke engines and 106,000 tons per year for two-stroke engines. The report also estimated 9,970 tons per year of NOx as NO2 for four-stroke engines and 344 tons per year for two-stroke engines, with particulate estimates of 1,400 and 1,500 tons per year respectively.
Small Engine Emissions Trends Across Technology and Regulatory Phases
EPA’s NONROAD2008a documentation reports model emission factors in grams per horsepower-hour, not direct fleet totals. Values are comparable within their stated class, pollutant, phase, and technology, but they should not be merged across unlike categories. EPA, “Exhaust Emission Factors for Nonroad Engine Modeling — Spark-Ignition: Report No. NR-010f”
| Class I nonhandheld factor | Baseline technology | Phase 3 technology |
|---|---|---|
| HC, g/hp-hr; engines under 225 cc | 207.92, two-stroke | 3.80, overhead-valve |
| CO, g/hp-hr; engines under 225 cc | 485.81, two-stroke | 238.45, side-valve |
| NOx, g/hp-hr; engines under 225 cc | 0.29, two-stroke | 1.04, side-valve |
| PM, g/hp-hr; engines under 225 cc | 7.7, baseline | 0.04, overhead-valve |
Within Class I, the documented HC factors differ sharply between the baseline two-stroke category and Phase 3 overhead-valve category. The PM entries likewise describe different technology categories, so the table shows a technology-and-phase comparison rather than a measured before-and-after fleet outcome.
For Class II nonhandheld engines over 225 cc, EPA lists a baseline overhead-valve HC factor of 5.20 g/hp-hr and a Phase 3 overhead-valve factor of 3.17 g/hp-hr. The corresponding CO factors are 408.84 g/hp-hr for the baseline category and 321.94 g/hp-hr for Phase 3.
Small Engine Emissions Statistics for Handheld and Nonhandheld Classes
Handheld and nonhandheld classes have distinct engine-size and technology definitions. Class IV covers handheld engines over 20 cc and under 50 cc, while Class V covers handheld engines over 50 cc.
| Handheld class and factor | Baseline | Phase 2 |
|---|---|---|
| Class IV HC, g/hp-hr | 261.00, two-stroke | 25.83, four-stroke |
| Class IV CO, g/hp-hr | 718.87, two-stroke | 141.69, catalyst |
| Class IV PM, g/hp-hr | 7.7, baseline | 0.06, four-stroke |
| Class V HC, g/hp-hr | 159.58, two-stroke | 47.98 |
| Class V CO, g/hp-hr | 519.02, baseline | 283.37 |
The Class IV entries show the highest listed baseline CO factor in this handheld comparison at 718.87 g/hp-hr, alongside a Phase 2 catalyst factor of 141.69 g/hp-hr. Its PM factors are 7.7 g/hp-hr for the baseline entry and 0.06 g/hp-hr for the Phase 2 four-stroke entry.
For Class V handheld engines over 50 cc, the HC factor is 159.58 g/hp-hr for the baseline two-stroke category and 47.98 g/hp-hr for Phase 2. CO is documented at 519.02 g/hp-hr for baseline and 283.37 g/hp-hr for Phase 2.
EPA’s later MOVES3 methodology adds context but not a direct fleet total: nonroad factors combine running and cold-start emissions and do not provide separate idle, start, and hot-stabilized factors. The report also notes that some nonroad toxic factors use surrogates or limited equipment-type data, and that technology, fuel subtype, and catalyst status affect composition. EPA, “Speciation Profiles and Toxic Emission Factors for Nonroad Engines in MOVES3”
Seasonal and Regional Small Engine Emissions Patterns
The historic EPA seasonal model allocated annual emissions across northern, central, and southern urban/suburban and rural cells. It is a simplistic modeled allocation, not monitoring data, and is based on a 1970-era model rather than a current inventory. EPA, “Exhaust Emissions From Uncontrolled Vehicles and Related Equipment using Internal Combustion Engines”
The model assigned 0% of annual emissions to December-February in each of the northern, central, and southern urban/suburban cells shown. For June-August, it assigned 24.65% to the central urban/suburban cell and 15.31% to the central rural cell.
The central urban/suburban subtotal was 51.81% of annual nationwide emissions in the model. The historic report rounded its regional summary to about 52% for the central United States, about 6% for the northern United States, and about 42% for the southern United States.
Those regional shares describe the model’s allocation of the historic national estimate, not measured concentration levels or a current state-by-state emissions inventory. Seasonal and regional comparisons therefore need to retain both the model period and the urban, suburban, or rural cell definition.