In the short term, elevated pollution causes coughing, throat and eye irritation, shortness of breath and reduced exercise capacity; it can trigger asthma attacks and, on bad days, raise hospital admissions for heart and lung problems. The effects begin within hours of exposure, which is why same-day AQI guidance is useful.
Over years, breathing polluted air is linked to higher rates of asthma, chronic obstructive pulmonary disease, heart disease, stroke and lung cancer. The World Health Organization estimates that ambient air pollution contributes to millions of premature deaths every year, making it one of the largest environmental health risks worldwide.
Risk is not evenly shared. The “sensitive groups” the AQI singles out — children, adults over 65, pregnant people and anyone with asthma, heart disease or lung disease — feel effects at lower concentrations and should act one category earlier than the general public.
WHO’s 2021 guidelines set a recommended annual PM2.5 limit of 5 µg/m³ and a 24-hour limit of 15 µg/m³ — stricter than many national standards — reflecting evidence that there is no threshold below which particulate pollution is provably safe.