4 Harmful effects

A. On human health

• Direct inhalation of NO₂ and SO₃/SO₂ (and acid aerosols) irritates the respiratory tract and increases the likelihood of asthma attacks, bronchitis and pneumonia.
• Ground-level ozone (O₃) is formed secondarily from the reaction of nitrogen oxides and volatile organic compounds in the presence of sunlight (Module 10). It harms the lungs and reduces respiratory capacity.
• Fine particles formed from SO₂ and NOₓ (sulfates, nitrates) penetrate deep into the lungs and bloodstream and cause heart and lung diseases.

B. On plants

• Direct damage to leaves. Gases such as SO₂ and NO₂ enter through the stomata, causing oxidation reactions that directly damage leaf tissue and deform the stomata.
• Reduced photosynthesis. Damaged chlorophyll and clogged stomata decrease the absorption of CO₂ and so reduce the production of glucose.
• Nutrient depletion. Acid rain leaches essential salts (such as calcium and magnesium) from the soil, lowering its fertility, and releases toxic aluminium ions, so plants lack the essential elements for growth.
• (Also acid rain acidifies lakes and rivers, killing fish eggs and sensitive species.)

C. On buildings and infrastructure

• Acids react with limestone (CaCO₃) and marble, causing corrosion that damages monuments and historic buildings:

  H₂SO₄ + CaCO₃ → CaSO₄ + CO₂ + H₂O

  (Check: Ca 1; S 1; C 1; O 4 + 3 = 7 = 4 + 2 + 1; H 2 = 2 ✔.) The calcium sulfate formed is slightly soluble and flakes off, so stone surfaces crumble. Metals (iron, bronze statues) also corrode faster; Egypt's limestone monuments are especially vulnerable.

Why does this threaten the sustainability of ecosystems and of humans? Biological productivity (forests, water bodies, wild plants) declines, affecting food and resources; there is a loss of biodiversity from the death of sensitive species or the disruption of their functions; the quality of air and water falls; and societies bear health and economic burdens (health care, building repairs, loss of resources).