TAYLOR, Texas — The first three parts of this investigation established the geography, the combustion equipment being proposed and how pollution would disperse beyond the Project Mustang property line.
The current submittal identifies 84 to 112 gas-fired units at 12.5 MW each. If every listed unit were installed, that schedule implies roughly 1,050 to 1,400 MW of aggregate nameplate capacity. The project also contemplates diesel backup and black-start generation.
The unanswered question is the one that matters most to people already living around the site: what happens if the concentrations reaching those homes are not trivial?
There is no evidence today that Project Mustang will expose nearby residents to unsafe pollution levels. But there is also no public project-specific dispersion analysis showing what concentrations the entire campus would produce at those homes.
More than 100 combustion sources would operate as one campus
EPA regulates stationary reciprocating engines because their exhaust can contain formaldehyde, acetaldehyde, acrolein, carbon monoxide, nitrogen oxides, volatile organic compounds and particulate matter.
Each Mustang engine would have to satisfy the rules applicable to that equipment. But nearby residents would not experience one engine in isolation. Their ambient air would reflect the combined contribution of the operating campus, layered onto existing background pollution.
That is why the eventual emissions inventory and dispersion model matter more than a statement that an individual engine meets an equipment standard.
NOx does not stop at the stack
Nitrogen oxides are among the pollutants that deserve close attention. EPA says stationary engines emit NOx and associates pollutants from these engines with respiratory problems, particularly for groups including children, older adults and people with asthma.
NOx can also contribute to formation of ground-level ozone and secondary fine particles after it enters the atmosphere. The Mustang question is therefore how much the full campus would emit, what controls would be used and what the modeled contribution would be at surrounding residences.
Fine particles and air toxics
Fine particulate matter, or PM2.5, can be emitted directly or form in the atmosphere from precursor gases. EPA associates PM2.5 exposure with respiratory and cardiovascular effects.
For reciprocating engines, another pollutant deserves particular attention: formaldehyde. EPA's stationary-engine rules specifically address formaldehyde along with hazardous air pollutants including acetaldehyde and acrolein.
That does not establish a health effect from Mustang. It establishes what needs to be measured. The public record should eventually identify source-specific emission rates, expected controls, short-term and annual emissions, and modeled concentrations at residential receptors.
Diesel backup adds another layer
Project Mustang also proposes diesel backup and black-start equipment. Those sources may operate less frequently than the primary gas system, but they still belong in the accounting.
The important questions include how many diesel generators Mustang would install, their size, testing schedules, whether multiple units could run simultaneously, and how extended emergency operation would be treated in the air analysis.
The concern is cumulative exposure
Residents would not encounter NOx in isolation one day and formaldehyde the next. Real-world air contains mixtures, and Mustang's contribution would exist alongside regional ozone, traffic, agriculture and other sources.
That does not mean the combined mixture will be dangerous. It means the project should be evaluated as a whole. The final technical record should tell the public what the entire campus contributes at the places people actually live.
What if the model identifies a problem?
If modeling identifies a nearby residence where a pollutant approaches an applicable standard or health-based screening level, design changes may still be possible: moving generation equipment, changing stack locations or parameters, improving controls, imposing operating limits or altering how many engines may run simultaneously.
Those options are easier to evaluate before major infrastructure is in the ground. Rigorous modeling can identify potential problems while the project can still be changed.
xAI shows how quickly the debate can become real
xAI's private-power development in Southaven, Mississippi, provides a useful cautionary example — not because it proves what Mustang will do, but because it shows how disputes over large behind-the-meter generation can develop.
In April, the NAACP sued xAI and a subsidiary, alleging that 27 gas turbines at the Colossus 2 data center had been operated without required federal air permits. Reuters reported that environmental groups estimated those turbines could emit substantial amounts of NOx, carbon monoxide, fine particulate matter and formaldehyde annually.
Those are estimates advanced by groups challenging xAI, and the lawsuit does not establish that Mustang would produce comparable emissions or health effects. It does show that cumulative emissions, source classification and permitting can become consequential questions once equipment is operating.
Compliance would matter — but it would not mean zero pollution
If Mustang's eventual TCEQ review shows concentrations comfortably below applicable standards at surrounding residences, that would be important evidence in the project's favor and should be reported plainly.
But compliance does not mean zero emissions or zero risk. It means the modeled project satisfies the standards and criteria being applied. The reverse is equally important: identifying NOx, PM2.5 or formaldehyde in engine exhaust does not establish harmful residential exposure. Concentration and dose matter.
What residents should eventually be able to inspect
- A complete engine-by-engine emissions inventory.
- Formaldehyde and other hazardous-air-pollutant calculations.
- Maximum hourly and annual NOx emissions.
- Diesel-generator emissions and operating assumptions.
- Final stack locations and parameters.
- Residential receptor locations.
- Cumulative dispersion-model results for the full campus.
- Applicable health-risk screening used by regulators.
Taylor should not have to guess
“Clean natural gas” does not answer the exposure question. Neither does simply listing pollutants with known health effects. The meaningful question is measurable: how much reaches the homes?
If the eventual analysis predicts very low residential concentrations, it can provide meaningful reassurance. If it identifies a problem, there may still be time to require changes.
There is no responsible basis today to say Project Mustang will make nearby residents sick. There is equally little basis to dismiss the health question merely because individual engines can be permitted.
More than 100 combustion sources could operate on one campus near existing residences. The pollutants involved are regulated because, at sufficient exposure, they can affect human health. The scale makes cumulative exposure a legitimate question, and until the project-specific numbers are public, the answer remains unknown.
Part 5 will examine the specific protections Taylor and state regulators can require before Project Mustang moves forward — and which protections should be enforceable rather than voluntary.