Why does the TVOC level slowly go up over time?

Aside from renovating or introducing new furniture, there are many everyday factors and sources that could cause TVOC levels to rise throughout the day. The U.S. Environmental Protection Agency estimates TVOC levels are consistently higher indoors (up to 10 times) than they are outdoors. This is mainly due to the high ambient temperatures - highly correlated with off-gassing rates - and lower ventilation.

Common sources of TVOCs are:

• Cleaning products
• Paints
• Aerosol sprays
• Human bio-effluents — emitted from human breath. Emissions are higher in enclosed and regularly occupied spaces.

For a full list, you can refer to the EPA's classification. 

When TVOC levels are high, you should increase ventilation in your environment, either by opening doors or windows or turning on fans and filtration system(s).