Step 1: Understanding the Concept: 
This question asks to identify the environmental factor that the World Health Organization (WHO) considers the most significant threat to human health globally. 
Step 2: Detailed Explanation: 
According to numerous reports and official statements by the World Health Organization, air pollution is the single largest environmental health risk. WHO data indicates that the vast majority of the global population breathes air that exceeds WHO guideline limits, containing high levels of pollutants. Exposure to air pollution (both ambient/outdoor and household/indoor) is a major cause of mortality and morbidity, leading to millions of premature deaths each year from noncommunicable diseases such as stroke, heart disease, chronic obstructive pulmonary disease, and lung cancer, as well as acute respiratory infections. While water, noise, and land pollution are also serious health risks, the sheer scale and severity of the impact of air pollution make it the top priority for the WHO. 
Step 3: Final Answer: 
The World Health Organization identifies air pollution as the world's top environmental health risk.