Covid aside, a reading of almost 6000 ppm in the bus is extremely high, at this level there is a clear impact on cognitive function.

See this study[1] in Nature that measured the effects of CO2 at 1000, 3000, and 5000 PPM. Quote: "For the Psychomotor Vigilance Test, the probability of achieving an accuracy score of >90% decreased in a dose–response like fashion from 79.5%, 74.7%, 73.4%, to 64.0% for 600, 1200, 2400, and 5000 ppm, respectively".

Another study[2] tried to establish the effect on response time, finding that "For every 500ppm increase, we saw response times 1.4-1.8% slower, and 2.1-2.4% lower throughput."

For a more pop-sci take on this, see this Tom Scott video where a subject is placed inside of an airtight chamber where the CO2 gradually increases, with a major impact on their brain function: https://www.youtube.com/watch?v=1Nh_vxpycEA

[1] https://www.nature.com/articles/s41526-019-0071-6

[2] https://iopscience.iop.org/article/10.1088/1748-9326/ac1bd8