Poor ventilation can potentially become extremely hazardous for the health of humans. When combined with reasonable moisture levels (poor ventilation often leads to raised moisture levels), moulds such as Aspergillus Niger can thrive - their fungal spores can cause breathing problems. Moulds and other fungi are the most likey cause of respiratory distress in moist, poorly ventilated environments.
Bacterial infections such as Legionnaires disease (Legionella pneumophila) can become rife in poorly ventilated areas, particularly in areas of standing water (such as, ironically, broken air conditioners)
Poor ventilation can also lead to non-biological health problems. Carbon Monoxide poisoning is often attributed to poor maintenance of boilers and other fossil fuel burning equipment COMBINED with poor ventilation. Aerosols used in similar environments can also prove fatal.
It is a very broad question, and the potential health problems are linked to the local environment of your domicile. If you are worried that inadequate ventilation may be causing health problems, get the area surveyed.
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