the mercury content of the new T8 lamps remained the same; however, the average life of the bulb increasedJust Short Explanation: The decrease in the average picogram per lumen hour indicates an improvement in the environmental impact of the lamps. This can happen if the mercury content remains constant while the lifespan of the lamps increases, resulting in less mercury used over time per unit of light produced.
References: The information about T8 lamps indicates that newer models are designed to be energy-efficient with a longer lifespan while maintaining a low mercury content, which would contribute to a lower picogram per lumen hour ratio12.