How does natural ventilation provide fresh air?

Natural ventilation provides fresh air primarily through open doors and windows. This method allows outdoor air to flow directly into indoor spaces, facilitating air exchange. When doors and windows are open, the pressure differences created by wind, temperature variations, and indoor-outdoor air quality allow fresh air to enter while stale air can exit. This passive system relies on the natural elements and does not require mechanical assistance or energy, making it an environmentally friendly option for improving indoor air quality.

In contrast, the other options involve mechanical systems or devices that typically require energy to operate, which does not align with the principles of natural ventilation. Electric fans, air conditioning units, and exhaust fans are all forms of mechanical ventilation, thus they don't contribute to the concept of natural air movement and fresh air supply through unobstructed openings like doors and windows do.