Final answer:
Workers must ask for assistance when the load is too heavy or large to handle on their own, or if it poses a risk of injury.
Step-by-step explanation:
In the context of manual handling, a worker must ask for assistance when the load is considered to be too heavy or too large for them to handle on their own, or if it poses a risk of injury. The weight and size of the load can vary depending on the individual's strength and capabilities, but generally, it is recommended to seek assistance for loads that exceed 50 pounds or are larger than what can be comfortably held and carried.