The WHO recognizes that modifiable environmental factors contribute significantly to disease, injury, and death worldwide. Environmental hazard categories can be broken down in a variety of ways, but here is one simple breakdown: physical, chemical, and biological hazards. All of these hazards can interact, sometimes in unpredictable ways, which increases the complexity of public health issues. Of these three groups, biological hazards remain the major risk in low-income nations, the areas where death rates are the highest.

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