Asthma patients are 1.5 to four times more likely to suffer an asthma attack if they live in an area with plentiful or large fracking wells, according to a new study which sheds negative light on the already-controversial drilling method.

Researchers from Johns Hopkins University studied 35,508 electronic health records from patients who were treated between 2005 and 2012 at the Geisinger Clinic in Pennsylvania – a state which saw 6,200 wells drilled between the mid-2000s and 2012.

Within the electronic records, the researchers identified 20,749 mild asthma flare-ups, with doctors prescribing new drugs to help patients manage symptoms. An additional 1,870 patients suffered cases of moderate worsening, visiting the emergency department. A total of 4,782 patients were hospitalized for severe exacerbations.

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The study found that asthma patients living closest to numerous or large active wells were found to be 50 percent more likely than those living further away to have severe flare-ups during the site preparation stage – one of four phases of unconventional natural gas development (UNGD) identified by the researchers.

Asthma patients were even more at risk during the production state, which can last several years. The study found that those living close to bigger or more fracking sites were four times more likely to have mild flare-ups during that stage.

However, the study has its limitations. In particular, the electronic health records did not have information on patients' occupations and only reflected their most recent addresses, not accounting for previous residential moves.

The researchers have called on further studies to investigate the link between fracking and asthma.

“Residential UNGD activity metrics were statistically associated with increased risk of mild, moderate, and severe asthma exacerbations. Whether these associations are causal awaits further investigation, including more detailed exposure assessment," the authors concluded.

The study was published in the Journal of the American Medical Association on Monday.

Fracking is a drilling technique that extracts oil and gas from the ground by blasting powerful, high-pressure water, sand, and chemicals into shale rock, fracturing it, and allowing oil and gas to be accessed.

The process is highly controversial, with environmentalists claiming it causes pollution and earth tremors. Critics also say that potentially carcinogenic chemicals used in fracking may escape and contaminate groundwater around the drilling site.