Some tiny things can make us sick. 
Some tiny things can make us sick. 
A biohazard is a substance that can harm living things. 

A biohazard is a biological substance that can harm living things. 
Safety signs use specific colors to tell us how much danger exists. A red sign with a black symbol means danger. This type of biohazard can cause death. An orange sign means a warning that death may happen. A yellow sign means caution for things that cause injury. Finally, a green sign is just a notice for things like cleaning.
People have used these special symbols for a long time. A man named Charles Baldwin created the biohazard symbol. He was an environmental-health engineer in 1966. He worked for the Dow Chemical Company. He was designing products to keep things contained.
Scientists group these hazards into four main levels. 

Working in a high-level lab requires very special gear. 
A biological hazard, often called a biohazard, is a substance that threatens the health of living organisms. These hazards primarily target humans, but they can also harm animals and other living beings.
The biohazard symbol is a key part of these warnings. It was developed in 1966 by Charles Baldwin. Baldwin was an environmental-health engineer for the Dow Chemical Company. He created the symbol while working on containment products. 
Color coding is used to communicate different levels of risk quickly. A black biohazard symbol on a red background with a white base is used for DANGER signs. This indicates a hazard that will cause death. An orange background with a black symbol is used for WARNING signs. These indicate a hazard that may cause death. A yellow background with a black symbol is used for CAUTION signs. These identify hazards that will cause injury but not death. Finally, a green background with a black symbol is used for NOTICE signs. These are for non-injury messages, such as lab hygiene or cleanup policies.
When biological substances are moved, they are classified by UN numbers. Category A, UN 2814, refers to infectious substances that affect humans. These can cause life-threatening or fatal disease in healthy humans or animals. Category A, UN 2900, refers to infectious substances that affect animals only. These can cause permanent disability or death in animals. Category B, UN 3373, includes biological substances used for diagnostic or investigative purposes. There is also Regulated Medical Waste, UN 3291. This is waste derived from medical treatment or biomedical research.
The United States Centers for Disease Control and Prevention (CDC) uses four Biosafety Levels, or BSL. Level 1 is the minimum risk level. It includes bacteria like Escherichia coli and viruses like chickenpox. Precautions at this level are minimal, usually involving gloves and facial protection. Level 2 includes bacteria and viruses that cause mild disease or are hard to contract through the air. Examples include hepatitis, mumps, and measles. Routine diagnostic work can be done safely at a BSL-2 facility.
Level 3 involves agents that can cause severe or fatal diseases. However, vaccines or treatments usually exist for these. Examples include anthrax, tuberculosis, and the SARS coronavirus. Level 4 is the most extreme risk level. 
Working in a Level 4 laboratory requires intense safety measures. Scientists must wear a positive pressure personnel suit. This suit has its own separate air supply to prevent contamination. 
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