Which of the following is a characteristic of biosafety level 4 (BSL-4)?

Biosafety Level 4 (BSL-4) is designated for work with the most dangerous pathogens that pose a high risk of aerosol transmission. These pathogens are often lethal and can cause serious diseases in humans. Facilities operating at BSL-4 are equipped with rigorous containment measures, including negative pressure airflow and specialized clothing, in order to protect personnel and prevent any potential release of pathogens into the environment.

Working with known human pathogens with a high risk of aerosol transmission is a defining characteristic of BSL-4, emphasizing the need for extreme precautions. The other options do not accurately describe BSL-4: handling well-characterized agents typically refers to lower biosafety levels with less stringent requirements, specific training is mandatory for BSL-4 due to the hazards involved, and laboratory access is restricted to ensure safety, as opposed to being open to all personnel.