Engineering Control Recommendations for Building Polytunnels that Stay Cooler During High Temperatures
Polytunnels (also known as hoop houses) are structures made of a metal or PVC skeleton covered with a sheet of thick, clear plastic. Polytunnels may be hotter inside than outside because sunlight passes through the plastic sheeting, heating up the air inside. Because polytunnels may be hotter inside than outside, they are often considered an outdoor environment by Cal/OSHA.
The temperature inside a polytunnel can vary depending on its size, location, and time of the year. But the temperature inside polytunnels is often higher than outside of polytunnels. Heat inside of polytunnels may even become dangerous for both plants and people. However, polytunnels can be built in a way that prevents the temperature inside polytunnels from becoming too hot. As part two in our three-part series on heat illness prevention in polytunnels, we recommend engineering controls for lowering the temperature inside polytunnels.
Engineering Control Recommendations
Construction
- Decrease the length of polytunnels to allow for more airflow through the tunnels.
- Position polytunnels so wind blows through the tunnel.
- Build taller polytunnels to allow heat to rise above the heads of farmworkers.
Ventilation
- Leave the ends of polytunnels open to allow for a breeze to move through the tunnel.
- Roll up plastic on the sides of polytunnels when workers are present to increase air flow.
- Install fans to move air through polytunnels.
Shading
- Add shade cloth on top of the plastic on hot days to reduce the temperature inside the polytunnel.
- Use a shade net (also called a Malla) over the tunnel structure to provide shade and lower the temperature.
- Apply white chalk to the plastic to reflect sunlight, which will decrease temperatures inside the polytunnel.
In addition to the above recommendations, consider if polytunnels can be eliminated—this is the most effective form of hazard control in the hierarchy of controls. In some cases, crops may not need to be grown in a polytunnel. This would eliminate the risk of farmworkers facing higher temperatures in polytunnels.
Resources
- View the seminar on YouTube in English or Spanish.
- Read article one in our three-part series: Can working in polytunnels affect farmworker health and safety?
This article provides an overview of heat illness prevention recommendations specific to polytunnels as part of our March 2024 seminar with speakers Isabella Kaser, Public Health Institute; Maripaula Valdes-Berriz, UC Division of Agriculture and Natural Resources; and Maureen McGuire, CEO of Ventura County Farm Bureau in collaboration with Roots of Change.
This article is funded by the California Occupational Health Branch of the California Department of Public Health.