Describe the physical pathway by which radon gas typically enters an indoor environment from the ground.

Radon gas originates from the natural radioactive decay of uranium, which is commonly found in soil and rock formations beneath and around buildings. This continuous process releases radon, an invisible, odorless, and tasteless gas, into the surrounding ground. As a gas, radon can move through the tiny interconnected spaces, or pores, within the soil and rock. This movement occurs primarily through two mechanisms: diffusion and pressure-driven flow. Diffusion is the natural tendency of gas molecules to spread out from an area of higher concentration to an area of lower concentration. However, the most significant mechanism for radon entry into indoor environments is pressure-driven flow, also known as convection, which is essentially the movement of gas due to differences in air pressure.Buildings typically create a lower air pressure indoors compared to the pressure in the surrounding soil beneath the foundation. This phenomenon, known as a "negative pressure differential," acts like a mild vacuum, actively drawing soil gas, including radon, into the building. This negative pressure can be exacerbated by the "stack effect," where warmer, less dense indoor air rises and exits through upper parts of a building, causing cooler, heavier air from the lower levels, including soil gas, to be drawn in. Additionally, exhaust fans, clothes dryers, furnaces, and other heating, ventilation, and air conditioning (HVAC) systems can further depressurize the indoor environment, increasing the draw of soil gas.Radon enters the indoor environment through various openings and pathways in the building's foundation that are in direct contact with the soil. Common entry points include cracks in concrete slabs, whether they are hairline or more significant, and openings in hollow-block foundation walls. The joint where the concrete floor slab meets the foundation wall is another frequent entry point due to potential gaps. Unsealed or poorly sealed utility penetrations, such as gaps around water pipes, sewer lines, electrical conduits, and fuel lines where they pass through the foundation, provide direct access. Open sump pits, which collect water, are particularly effective conduits if not properly sealed. Even seemingly solid concrete can have microscopic pores that allow some radon to pass through. In homes with dirt-floor crawl spaces or earthen basements, the uncovered soil presents a very direct and significant pathway for radon to enter the living space. Once inside, if ventilation is limited, radon can accumulate, especially in lower levels like basements and crawl spaces, reaching elevated concentrations.