What is “Zero Emissions”? A Look Under the Hood of the Infrastructure Bills

Wiley Rein LLP

What does “zero-emissions” mean, exactly? Even when the focus is on tailpipe emissions alone (and not on upstream emissions or production and the supply chain), multiple definitions abound. And sometimes, when there are multiple visions, confusion follows.

The infrastructure bills under consideration in the U.S. Congress provide a good example of this phenomenon.

The House Ways and Means Committee[1] and the House Committee on Energy and Commerce[2] marked up and advanced the Build Back Better Act last week. As passed by both committees, the bill appropriates billions of dollars for grants, loans and other programs related to “zero-emission” transportation like buses, vehicle infrastructure, port equipment, and heavy-duty vehicles. The infrastructure bill passed by the Senate in August[3] also contains grants for clean and zero-emission buses.

Even though they share a focus on tailpipe emissions, the proposals use four different definitions of “zero emission(s),” some of which are in tension with one another. This proliferation of definitions could lead to unnecessary confusion down the road, particularly with respect to hydrogen fuel cells.

Definition A: Section 30101 – zero criteria pollutants, zero greenhouse gases. The Energy and Commerce markup of the Build Back Better Act has one fairly straightforward definition of “zero emission” in a section on $5 billion in grants and rebates for purchase of zero-emission heavy-duty vehicles.[4] In this definition, zero-emission vehicle means “a vehicle that has a drivetrain that produces, under any possible operational mode or condition, zero exhaust emission of (A) any air pollutant that is listed pursuant to section 108(a) (or any precursor to such an air pollutant); and (B) any greenhouse gas.’’

This is substantially the same the definition found in the California Air Resources Board Zero Emissions Vehicle (ZEV) regulation.[5] It is also substantially the same as CARB’s definition of ZEV in its draft Advanced Clean Fleets regulation released this summer, which will apply to medium- and heavy-duty vehicles.[6]

Looked at in isolation, this definition seems clear enough. However, seen in conjunction with the next definition in the Build Back Better Act, some problems emerge.

Definition B: Section 30102 – zero criteria pollutants, and zero greenhouse gases except water vapor. A second definition in the Energy and Commerce markup of the Build Back Better Act is included in a section on $3.5 billion in grants to reduce air pollution in ports. In this section, “zero emissions port equipment and technology” is defined in relevant part as “any equipment or technology that ‘produces zero emissions of any air pollutant that is listed pursuant to section 108(a) (or any precursor to such an air pollutant) and any greenhouse gas other than water vapor’ (emphasis added).

The inclusion of “water vapor” in this definition—and not in Section 30101 or anywhere else for that matter—raises some questions.

First, why reference water vapor at all? Water vapor is commonly understood to be a greenhouse gas, albeit one that EPA or CARB do not regulate at the tailpipe.[7] The agencies may be refraining from regulation here, for example, because tailpipe emissions of water vapor are insignificant compared to natural sources such as evaporation from oceans, and it is unclear whether increases in atmospheric water vapor concentrations cause (or are instead caused by) global warming.[8]

In this context, the bill likely includes a reference to water vapor to ensure that fuel cell technology can be classified as “zero emissions” even though it produces emissions of water vapor.[9] Without such an exception, and standing alone, a definition that disqualifies a vehicle from being “zero emissions” if it produces “any greenhouse gas” could be read to exclude fuel cells.[10]

Second, why reference only water vapor in this section and not in any other section that deals with zero emission technology? This question is a little more difficult to answer. There is nothing special about technologies used in and around ports that would counsel in favor of making doubly sure that fuel cells qualify as zero emissions for purposes of the grants provided in Section 30102.

In some contexts, a specific reference to water vapor might not be necessary, because other provisions of the relevant regulatory or statutory scheme indicate that fuel cells should be considered zero emissions. For example, although it uses Definition A and does not exempt water vapor, the CARB ZEV regulation defines “hydrogen fuel cell vehicle” as “a ZEV that is fueled primarily by hydrogen.”[11]

However, in many cases—such as Section 30101—the relevant statutory scheme does not contain any specific references to fuel cells.

As a legislative drafting matter, the problem could probably be avoided by using one of the other definitions of “zero emission” from the infrastructure bills.

Definition C: Section 135107 – Clean Fuel Fleet or CARB ZEV. The Ways and Means Committee markup[12] includes a section on exempt facility bonds for zero-emission vehicle infrastructure. This section defines a “zero-emissions vehicle” as “(A) a zero-emission vehicle as defined in section 88.102–94 of title 40, Code of Federal Regulations, or (B) a vehicle that produces zero exhaust emissions of any criteria pollutant (or precursor pollutant) or greenhouse gas under any possible operational modes and conditions.”[13]

This definition is a composite of two existing regulatory definitions.

Part A is from EPA’s Clean Fuel Fleet program, which encourages certain heavily populated municipalities to employ lower-emitting trucks and other vehicles. The applicable definition, 40 CFR § 88.102-94, states: “Zero-Emission Vehicle means any light-duty vehicle or light-duty truck conforming to the applicable Zero Emission Vehicle standard, or any heavy-duty vehicle conforming to the applicable Zero-Emission Vehicle standard.”[14]

Part B is the current CARB ZEV definition.[15]

Although the Ways and Means Committee definition has two different parts and is somewhat complex, it has the advantage of using existing language with which regulated and affected entities are presumably already familiar. Moreover, although Part B of the regulation could theoretically be read to exclude fuel cells since they emit water vapor, Part A of the regulation arguably would allow fuel cells to qualify without much difficulty.

Definition D: “no carbon or particulate matter”. In a section that amends a law authorizing the Department of Transportation to make grants for buses and bus facilities,[16] the Senate infrastructure bill cross-references (without revision) yet a third definition of “zero emissions”: producing “no carbon or particulate matter.”[17]

Notably absent from this definition: any reference to the full suite of criteria pollutants, and any reference to greenhouse gases. It is at least theoretically possible that a technology that qualifies as zero emissions under this definition would fail under the other definitions, because it produces NOx emissions but no carbon or particulates. (Whether such a technology exists is a different question.) Note as well that this definition does not have the problems that might arise for fuel cells under Definition A. Because this definition does not reference greenhouse gases (only carbon), there is no need to exempt water vapor from the definition to ensure that fuel cells qualify.


The infrastructure bills are not yet final, of course, and there is still no guarantee that they will pass in their current form—or indeed in any form at all. And if these provisions do pass, federal agencies can likely find ways around any problems that might be posed by Definitions A and B for fuel cells, for example by relying on statutory definitions of the term “greenhouse gas.”

Given the billions of dollars in funding that are potentially at stake, however, supporters of new technologies need to pay close attention to the relevant statutory provisions and, eventually, be prepared to marshal the best possible arguments that their technologies qualify under whatever provisions become law.

Wiley is carefully monitoring these developments.

[1] https://waysandmeans.house.gov/media-center/press-releases/neal-applauds-committee-advancement-build-back-better-act

[2] https://energycommerce.house.gov/newsroom/press-releases/pallone-on-committee-passage-of-build-back-better-act

[3] See 117th Cong., H.R. 3684 (engrossed amendment Senate), https://www.congress.gov/117/bills/hr3684/BILLS-117hr3684eas.pdf

[4] Text available at https://docs.house.gov/meetings/IF/IF00/20210913/114039/BILLS-117-A-A000370-Amdt-1.pdf

[5] See Cal. Code Regs, tit. 13, § 1962.1.

[6] https://ww2.arb.ca.gov/sites/default/files/2021-08/210909draft100zev_ADA.pdf

[7] Some statutory provisions define “greenhouse gas” to exclude or potentially exclude water vapor. See, e.g., 42 U.S.C. § 17321(3) (definition of greenhouse gas that does not include water vapor); 49 U.S.C. § 5339(c)(1)(A) (“the term ‘direct carbon emissions’ means the quantity of direct greenhouse gas emissions from a vehicle, as determined by the Administrator of the Environmental Protection Agency”).

[8] See, e.g., https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives/its-water-vapor-not-the-co2.html, https://www.ncdc.noaa.gov/monitoring-references/faq/greenhouse-gases.php#h2o

[9] See, e.g., US Dept. Energy, Alternative Fuels Data Center, https://afdc.energy.gov/fuels/hydrogen_benefits.html (“Once produced, hydrogen generates electrical power in a fuel cell, emitting only water vapor and warm air.”).

[10] In a similar vein, the California Air Resources Board recently proposed amending its commercial watercraft regulations to specify that “zero emissions” means “no tailpipe exhaust emissions other than water vapor or diatomic nitrogen.” See California Air Resources Board, Proposed Amendments to the Commercial Harbor Craft Regulation (Sept. 21, 2021) at 34, https://ww2.arb.ca.gov/sites/default/files/barcu/regact/2021/chc2021/appa.pdf (“‘Zero-Emission’ means a propulsion system, auxiliary power system, and/or vessel utilizing a zero-emission propulsion and auxiliary power system that has no tailpipe exhaust emissions other than water vapor or diatomic nitrogen from the onboard source(s) of power.”).

[11] See Cal. Code Regs, tit. 13, § 1962.2(i)(7). The California heavy-duty vehicle exhaust emissions standards take a different approach and specifically define “zero-emission powertrain" in relevant part as “an all-electric or hydrogen fuel-cell powertrain assembly.” Id. § 1956.8(i)(12).

[12] See https://waysandmeans.house.gov/sites/democrats.waysandmeans.house.gov/files/documents/NEAL_032_xml.pdf

[13] This definition is also used in 49 USC § 49136 (establishing a DOT program for zero emission airport vehicles and infrastructure).

[14] The Zero Emission Vehicle standard for light-duty vehicles or light-duty trucks, § 88.104–94, was designed in 1994 to be consistent with the California Air Resources Board Zero Emission Vehicle program that existed at the time. It has four components. To be certified as a Zero Emission Vehicle, an engineering analysis must show that (1) “The vehicle fuel system(s) must not contain either carbon or nitrogen compounds (including air) which, when burned, form [NMOG, CO, NOx, HCHO or PM],” (2) “All primary and auxiliary equipment and engines must have no emissions of [NMOG, CO, NOx, HCHO or PM],” (3) “The vehicle fuel system(s) and any auxiliary engine(s) must have no evaporative emissions in use,” and (4) “Any auxiliary heater must not operate at ambient temperatures above 40 degrees Fahrenheit.” § 88.104–94(g). The Zero Emission Vehicle standard for heavy duty vehicles is a little more straightforward in that it states a clear standard: “A heavy-duty zero-emission vehicle (ZEV) has a standard of zero emissions for nonmethane hydrocarbons, oxides of nitrogen, carbon monoxide, formaldehyde, and particulates.” § 88.105-94(f)(1). The regulation also contains a section that parallels Section 88.104-94(g) and specifies the conditions under which heavy-duty vehicles can be certified as ZEVs.

[15] See 59 Fed. Reg. 50,042, 50,050 (Sept. 30, 1994). California currently defines a Zero Emissions Vehicle as “a vehicle that produces zero exhaust emissions of any criteria pollutant (or precursor pollutant) or greenhouse gas under any possible operational modes or conditions.” Cal. Code Regs, tit. 13, § 1962.1(i)(18).

[16] See H.R. 3684(engrossed amendment Senate), Section 30018, amending 49 U.S.C. § 5339.

[17] 49 USC § 5339(c)(1)(G) (“the term ‘zero emission vehicle’ means a low or no emission vehicle that produces no carbon or particulate matter.”). The definitions of “zero emission vehicle” and “low or no emission vehicle” are defined in terms of each other, which is awkwardly circular. The same definition is used in 49 USC § 5312 (assistance for public transportation innovation).

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