Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Kairo has been climbing in recent years. All-time it's been given to 21,518 U.S. babies, the 291st most common name by sound today, peaking at 2,614 in 2022, with 2,611 in 2025.
By sound, Kairo also includes the spellings Cairo.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).