Great question! Very often - really any time it’s obvious that you’re referring to a measurement - including your example, both work (there is no real difference in meaning). The grammar is “low” (adjective)+ noun or “determiner + low (adjective) + level (noun) + of + noun”). I prefer “low + noun” because it’s more concise, but sometimes “low level of” helps clarity. Sometimes, “low level of” is needed to indicate that you’re making a measurement, e.g. I would only say “you have a high level of English”, not “you have high English”. I hope this helps!

If the noun can only be high or low, that is it can only be a numerical value, just use 'low'. Examples are: Salary Birth rate Temperature Income All of these are terms for numerical values only, so just write: "Women tend to earn lower salaries than men" "Low temperatures led to poor harvests" "Incomes are lower than they were five years ago" When a noun is not simply a numerical value, use 'level' to make it clear that the numerical value is the aspect being described. Examples are: Education Literacy Satisfaction Crime All of these can be described in ways other than numerically, so write. "Levels of literacy tend to be lower among girls than boys" "These days, people tend to have lower levels of life satisfaction than in the past. "Small towns generally enjoy low crime levels"