The phrase "are now harmful" is correct and usable in written English.
It can be used to describe a change in the nature or impact of something, indicating that it has become detrimental or damaging over time.
Example: "The chemicals that were once considered safe are now harmful to the environment."
Alternatives: "have become detrimental" or "are currently damaging".
Until well past the midpoint of the last century, the so-called science of painting restoration embraced practices that are now considered harmful and for the most part unnecessary, such as relining.
Worse yet, there's a good chance that you're set in anachronistic ways, hidebound with habits which are now considered harmful.
These songs of unity and nationalism that once served great causes and motivated us are now as harmful as the propaganda our tyrants feed us- they serve not as reminders of the raw political truth but as a temporary pat on the back for our shortcomings.
More importantly, the passive effects of smoking have been almost eliminated so that non-smokers are now protected from the harmful effects.
Bedding made from red cedar (Eastern or Western) and pine, both softwoods, were commonly used in the past, but these materials are now believed to contain harmful phenols (aromatic hydrocarbons) and oils.
Catharine MacKinnon, in "Only Words,O maintains that, having become unduly tolerant of things once considered harmful, we are now suffering from the effects of that permissiveness.
Harmful algal blooms are now a common occurrence around the world in these Anthropocene times because of fertilization of phosphorus and nitrogen for crop and animal production.
The key is to figure out when and for whom they will be helpful or harmful, and researchers are now exploring these questions.
"It is European action that put a stop to the devastating impact on our forests of acid rain, and we are now tackling air quality by cutting harmful emissions.
