"Tell us what is the the maximal safe level in PPM for CO2 in our atmosphere?"

<<Safe for what?>>

LOL! You are such a moron! From the very same article you quoted, we get this!

Don't put too much CO2!

Too much CO2 is bad to the plants. Too high CO2 level lowers plants' transpiration during photosynthesis: without or with less transpiration less nutritive solution is drawn thru the plant, thus less food enters the plant and growth slows down. Under too high CO2 level, necrosis spots (dead vegetal tissue, Figure 2) appear on leaves that may also roll into themselves. These dead tissue spots are a great food for bacteria and molds."

You can't win for losing!

it is known for a fact that carbon dioxide (CO2) concentration was much higher than it is now. Then, the CO2 concentration was certainly above 1000 parts per million (ppm). Actually, the average CO2 concentration in outdoors air is about 400 ppm on the planet (not really true at your location). Thus, plants enjoy and are stimulated by breathing air with a higher CO2 concentration. That’s why so many indoor gardeners enrich their garden with CO2 during photosynthesis to supply the plants with this essential building material. Through photosynthesis, the carbon in CO2 is extracted and takes part in the building of leaves, stems, flowers and fruits. Proper CO2 concentration from early growing to fructification allows for faster maturation and larger yield.http://www.novabiomatique.com/hydroponics-systems/plant-555-gardening-with-co2-explained.cfm