[unearth3d]

Well humans would suffer with 840ppm: e.g. Seppänen etal. 1999. Association of Ventilation Rates and CO2-Concentrations with Health and other Responses in Commercial and Institutional Buildings. Indoor Air 9: 226-252. Discuss sick building syndrome effects fell sharply <800ppm. Publ. CO2 levels are outside; within buildings even now rates >1000 common, as ambient rises so do within-building rates.

Even at our current levels eCO2 is harming our health: Rising CO2, Climate Change, and Public Health: Exploring the Links to Plant Biology https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2649213/ these are direct CO2 effects on plant physiology leading to negative human outcomes.

eCO2 has dozens of negative effect on natural (and engineered systems including concrete chemistry) systems (inc. us). There are no positives that I've seen yet.