What must be readily available outside of containment barriers to check air flow?

The correct answer is smoke tubes. Smoke tubes are used in industrial hygiene to visually assess airflow by releasing a harmless smoke that can indicate the direction and speed of air movement. This is particularly important in situations involving asbestos containment, as maintaining proper airflow is critical for ensuring that contaminants do not escape into surrounding areas.

In the context of maintaining safety during asbestos removal or abatement activities, smoke tubes can help workers determine if the containment structure is functioning properly and if negative pressure is being maintained. This visual indicator allows for immediate evaluation and necessary adjustments to be made, enhancing the safety protocols in place.

The other options, while useful in various contexts, do not serve the same purpose for checking air flow in real-time. Pressure gauges are important for monitoring pressure levels, but they do not provide the immediate visual feedback that smoke tubes do. Thermometers measure temperature, which is not directly related to air flow checks in this context, while humidity meters track moisture levels, which can also have indirect effects but are not designed for assessing air movement.