Final answer:
The worst pollutants in terms of health effects according to the WHO include particulate matter, ground-level ozone, and gases like NOx and SOx; these can cause serious respiratory issues and decrease oxygen delivery to organs.
Step-by-step explanation:
According to the World Health Organization (WHO), the worst pollutants in terms of health effects include particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Among these, particulate matter and ground-level ozone are considered the most widespread health threats. Nitrogen oxides (NOx) contribute to the formation of ground-level ozone and particulate matter, producing health issues such as asthma and bronchitis. Sulfur dioxides (SOx) also affect human health by forming fine sulfate particles that can be transported over long distances and inhaled deep into the lungs. Carbon monoxide (CO) is a colorless, odorless gas that can reduce oxygen delivery to the body's organs, causing harmful health effects and at very high levels, causing death.