Conservation policies that are usually pushed to protect the environment can also improve human health, a new study suggests.

Recent estimates attribute as much as 24% of the global burden of disease to poor environmental quality. However, researchers from Duke University suggest that there is insufficient evidence to demonstrate that environmental degradation harms human health.

To address this knowledge gap, researchers analyzed four years of data that included patterns of disease, climate, demographics, land use, and conservation policies in the Brazilian Amazon. The study, published in Proceedings of the National Academy of Sciences, expanded upon current literature that usually focused on the drivers of specific ailments by also factoring in the human ecology of these diseases. These include the impact of key policies in the Amazon, such as road construction and established protected areas.

"Many of these factors have previously appeared separately in disease models, but not all together in one model," lead researcher Subhrendu Pattanayak said in a statement. 

Researchers focused the study on the Brazilian Amazon because it had undergone rapid changes in land use, with nearly a fifth of the original forest cover converted to other land uses, and there have been substantial conservation efforts in this region. The study evaluated the impact of these factors on malaria, diarrhea, and acute respiratory infections (ARI).

"Our findings suggest that strictly protected areas may serve as a barrier to disease transmission," Pattanayak said.

Strictly protected areas, such as parks and reserves, were found to reduce the incidence of all three factors. However, there was mixed evidence on the influence of roads—it increased incidences of malaria, but decreased diarrhea and ARI cases. Finally, there was a higher incidence of malaria observed in ‘sustainable use’ protected areas. The team hypothesizes that roads may have reduced the rates of ARI and diarrhea because they improve access to healthcare, while sustainable protected areas may have had higher incidence of malaria because they increase exposure to the mosquitos that transmit the parasite.

"Although these relationships are complex, we believe that by protecting and preserving biodiversity, we could also derive the double win of public health benefits," Pattanayak said. 

Pattanayak and her research team acknowledge their analysis does not account for the full costs and benefits of different types of protected areas, mining, or roads. This will require data on diseases, sociodemographic factors, and policies over a longer period of time.

"Certainly the causal chain of environmental health is neither short nor simple," said Pattanayak. "But my colleagues and I believe our methods and this study help add important links in this chain. As such, it strengthens the claims of a recent Lancet Commission on Planetary Health that calls for protecting nature to achieve health outcomes."