
bedenklich
concerning
Worrying or alarming; giving cause for concern or doubt.
Die Ärzte bezeichneten seinen Gesundheitszustand als bedenklich.
The doctors described his state of health as concerning.
concerning / questionable
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concerning
Worrying or alarming; giving cause for concern or doubt.
Die Ärzte bezeichneten seinen Gesundheitszustand als bedenklich.
The doctors described his state of health as concerning.

questionable
Morally or ethically questionable; dubious.
Es ist bedenklich, wie das Unternehmen mit den Daten seiner Kunden umgeht.
It is questionable how the company handles its customers' data.
'Bedenklich' (concerning / questionable) is derived from the noun 'das Bedenken' (doubt, concern). Something that is 'bedenklich' therefore gives cause for doubts or concerns.
Ich habe Bedenken bei diesem Plan. Ich finde diesen Plan bedenklich.
I have concerns about this plan. I find this plan questionable.
The adjective is often used with verbs such as 'sein' (to be), 'wirken' (to seem/appear), or 'erscheinen' (to appear). Another very typical construction is 'jemand findet etwas bedenklich' (someone finds something concerning/questionable), used to express a personal assessment.
Ich finde es bedenklich, dass so viele junge Leute rauchen.
I find it concerning that so many young people smoke.
In the meaning 'besorgniserregend' (worrying/concerning), 'bedenklich' often describes a negative state or process. Common combinations are: 'eine bedenkliche Entwicklung' (a concerning development), 'ein bedenklicher Zustand' (a concerning condition), or 'ein bedenkliches Niveau' (a concerning level).
Der Wasserstand des Flusses hat ein bedenkliches Niveau erreicht.
The river's water level has reached a concerning level.
When moral doubts are involved, 'bedenklich' often describes actions or attitudes. Typical nouns here are: 'ein bedenkliches Verhalten' (questionable behavior), 'eine bedenkliche Methode' (a questionable method), or 'eine bedenkliche Aussage' (a questionable statement).
Die Methoden des Unternehmens sind rechtlich legal, aber moralisch bedenklich.
The company's methods are legally permissible, but morally questionable.
serious / seriously
Ernsthaft and bedenklich can both assess a health condition or a problem. Ernsthaft mainly describes the severity, while bedenklich emphasizes more strongly that something is cause for concern. Choose ernsthaft when assessing the condition and bedenklich when warning about it.

Die Verletzung ist bedenklich und macht den Arzt sehr besorgt.
The injury is concerning and is making the doctor very worried.
Die Verletzung ist ernsthaft und muss heute behandelt werden.
The injury is serious and must be treated today.