The ecological effects of acid rain are most clearly seen in aquatic environments, such as streams, lakes, and marshes. Acid rain flows to streams, lakes, and marshes after falling on forests, fields, buildings, and roads. Acid rain also falls directly on aquatic habitats. Most lakes and streams have a pH between 6 and 8, although some lakes are naturally acidic even without the effects of acid rain. Acid rain primarily affects sensitive bodies of water, which are located in watersheds whose soils have a limited buffering capacity (ability to neutralize acidic compounds). Lakes and streams become acidic when the water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it. In areas where buffering capacity is low, acid rain also releases aluminum from soils into lakes and streams, which is highly toxic to many species of aquatic organisms.
Forests also display the visible effects of acid deposition. Acid rain causes slower growth, injury, or death of forests, as well as soil degradation, which leads to reduced reproduction of plants. Acid deposition usually does not directly kill the trees, but simply damages their leaves, thereby reducing or stopping photosynthesis. These acids also cause the release of aluminum and other harmful materials from the soil, which are toxic to trees and other plants. Dry acid deposition compounds these problems by settling harmlessly on the trees, then washing to the ground when acid rain falls. This results in a higher concentration of acid (lower pH) being absorbed by the soil or washed into aquatic ecosystems.
Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. However, the pollutants that cause acid rain also damage human health. These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled into people's lungs.