Solar activity follows an approximately 11-year cycle, known as the solar cycle, which significantly impacts the level of cosmic radiation reaching the Earth. During periods of high solar activity, known as solar maxima, the sun emits increased solar wind. This solar wind, a stream of charged particles, interacts with cosmic rays, creating a protective shield that deflects some of these high-energy particles away from the Earth. As a result, the intensity of cosmic radiation reaching the Earth's surface is reduced during solar maxima. Conversely, during solar minima, when solar activity is low, the reduced solar wind allows more cosmic radiation to penetrate the Earth's atmosphere. This increased exposure can have implications for both the environment and human health, particularly for individuals at high altitudes or those involved in high-altitude or space travel, where the atmosphere's protective effect is diminished.

Radon is a significant health risk primarily due to its radioactive nature and its ability to accumulate in enclosed spaces like homes. Radon is a colourless, odourless gas produced from the decay of uranium in the Earth's crust. It seeps into buildings through cracks and openings, particularly in areas with uranium-rich soil. When radon decays, it emits ionizing radiation in the form of alpha particles. Prolonged exposure to high levels of radon increases the risk of lung cancer, especially in individuals who smoke. Radon contributes to background radiation as it is a ubiquitous component of the natural environment. However, its concentration varies geographically due to differences in soil composition. The health risk associated with radon emphasises the importance of monitoring indoor radon levels, particularly in areas known for high natural uranium content.

Background radiation levels can indeed vary significantly within a small geographic area, influenced by several factors. One primary factor is the composition of the local geology. Areas with higher concentrations of natural radioactive elements like uranium, thorium, and potassium-40 will have higher background radiation levels. Another significant factor is human activity, such as mining or industrial processes, which can concentrate or release radioactive materials. Additionally, local construction materials, which may contain varying levels of natural radioactivity, can also contribute to this variation. Environmental factors, like the presence of radon gas, which can accumulate in certain areas depending on soil and rock permeability, also play a crucial role. Finally, altitude can cause variations even within a small area; higher elevations are exposed to more cosmic radiation due to thinner atmospheric shielding.

Different materials and structures provide varying degrees of shielding against background radiation, primarily based on their composition and density. Dense materials like lead and concrete are commonly used for shielding against radiation due to their high atomic numbers, which effectively absorb and scatter ionizing radiation. The thickness of these materials also plays a crucial role; thicker barriers provide more effective shielding. For cosmic radiation, structures such as buildings and natural geographical features like mountains can offer some protection by absorbing a portion of the high-energy particles. However, lighter materials like wood and plastic provide minimal shielding against most types of ionizing radiation. In environments where radiation protection is critical, such as nuclear power plants or medical facilities, specialised materials and construction techniques are used to ensure adequate shielding and minimise radiation exposure.

Living organisms have evolved various biological processes and adaptations to cope with the natural levels of background radiation. One such adaptation is DNA repair mechanisms. Cells are constantly exposed to low levels of ionizing radiation that can cause DNA damage. To counter this, organisms have evolved complex DNA repair systems that identify and repair these damages, thus maintaining genetic integrity. Another adaptation is the production of antioxidants. These molecules can neutralise free radicals produced by ionizing radiation, thereby protecting cells from oxidative stress and potential damage. Additionally, some organisms, particularly those in high-radiation environments, have developed increased tolerance to radiation. For example, certain bacteria and fungi found in nuclear reactors or areas with high natural radiation demonstrate remarkable resistance, suggesting adaptations at the cellular or molecular level. These adaptations are ongoing areas of research, offering insights into radiation protection and potential applications in medicine and biotechnology.