Living near new oil and gas wells was linked to higher Valley fever risk in California

Coccidioidomycosis — Valley fever — is a fungal disease caught by inhaling Coccidioides spores that live in soil, and it is spreading as a warming, drying climate expands the dust-prone regions where the fungus thrives. A study in The Lancet Planetary Health, published in the September issue, adds a specific and previously unexamined risk factor: the development of new oil and gas wells near where people live [s1].

The hypothesis

The logic is a chain of physical steps. The spores sit in soil; activities that disturb soil and raise dust can aerosolise and carry the pathogen; and the oil and gas industry, heavily developed in some regions where the fungus is endemic, is a known dust source [s1]. Various health harms have been tied to oil and gas development before, but its effect on Valley fever specifically had not been investigated [s1]. This study set out to estimate the association between exposure to well development — wells in preproduction, the phase involving the most ground disturbance — and the risk of infection among nearby residents.

The design

The researchers used a case-crossover design, which compares each person's exposure during the period before they fell ill against their own exposure during control periods — a structure that automatically accounts for characteristics that don't change over time, since each case serves as their own comparison [s1]. They drew on reported coccidioidomycosis cases and records of oil and gas well development in Kern County, California, between 2007 and 2022 [s1]. Exposure was defined as preproduction wells within 5 km of a person's home, compared across hazard periods (the 49–139 days before onset) and control periods using conditional logistic regression [s1].

The scale of exposure in this region is striking. Of 909,282 Kern County residents, 658,108 (72.4%) lived within 5 km of at least one preproduction well, and 116,020 (12.8%) lived within 5 km of 23 or more preproduction wells within a single 90-day period [s1].

What it found

The odds of coccidioidomycosis were 12.5% higher (95% CI 5.8 to 19.6) in the 90 days following exposure to at least one preproduction well within 5 km of a person's home [s1]. There was also a dose-response pattern: the odds of infection rose by 0.7% (95% CI 0.4 to 0.9) for each additional preproduction well developed within that distance [s1].

The authors interpret this as support for a previously unrecognised association between well development and Valley fever transmission, plausibly driven by dust generation, and note that given how much oil and gas development the region has, the population-level impact could be large [s1].

How to weigh it

The case-crossover design is well suited to this question because it controls for stable personal factors, and the dose-response gradient strengthens the case that the association is not a fluke. But this is observational work, and it rests on proximity as a proxy for exposure: living within 5 km of a well is not the same as inhaling spores from it, and the study cannot confirm the dust-borne mechanism it proposes. Reported cases also undercount true infections, since many are mild or never diagnosed, and reporting patterns can vary in ways that track with geography.

What the study establishes is an association, with a plausible physical mechanism and a dose-response shape, between new well development and Valley fever risk in one heavily drilled, endemic county. It does not prove causation, and it is specific to Kern County. But it identifies a modifiable, human-made contributor to a disease whose range climate change is already widening — the kind of finding that points toward where dust-control and monitoring efforts might matter most.

Sources

• Oil and gas well development and coccidioidomycosis risk in Kern County, CA, USA: a case-crossover study — The Lancet Planetary Health, 2026-09-01