What is the chemical composition of normal acid rain, and what kinds of harm might acid rain bring to humans? Thanks?

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Rainwater with a pH value lower than 5.6 (the pH value of unpolluted rainwater) is known as acid rain.  The major components of acid rain are sulfuric acid solution (H₂SO₄) and nitric acid solution (HNO₃).

Sulfur dioxide is the primary cause of acid rain.  Sulphur dioxide causes acid rain because it produces dilute sulphuric(VI) acid with rainwater.

2SO₂(g) + O₂(g) + H₂O(ℓ) → H₂SO₄(aq)

The actual pathway is more complex.

Sulfuric(VI) acid is also formed when sulfur dioxide dissolves in rainwater.

SO₂(g) + H₂O(ℓ) → H₂SO₃(aq)

Nitrogen dioxide also cause the formation of acid rain.  Nitrogen dioxide causes acid rain because it produces dilute nitric(V) acid with rainwater and atmospheric oxygen.

4NO₂(g) + 2H₂O(ℓ) + O₂(g) → 4HNO₃(aq)

The actual pathway is more complex.

Harmful effects of acid rain on environment include :

• In water of pH less than 4.5, calcium metabolism in fresh water fish will be badly affected, leading to poor health and stunted growth.  As a result, diversity and population of some fresh water species will be reduced.

• In soil of pH less than 4.5, absorption of cations by plants will be badly affected, resulting in death of plants.

• Acid rain corrodes metals and accelerates the deterioration of building, rock and statue.