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.

'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.
dangerous
'gefährlich' and 'bedenklich' can both express that something is not good or not safe. 'gefährlich' clearly says that something can cause harm, such as injuries or serious consequences. 'bedenklich' is often a little weaker or more general: something causes concern, seems doubtful, or is morally questionable. Learners may confuse the words because both sound negative. When there is a real danger, 'gefährlich' is more precise. When something gives cause for concern or reflection, 'bedenklich' fits better.

Die Entscheidung des Vereins wirkt bedenklich, weil niemand die Gründe erklärt.
The club's decision seems questionable because no one explains the reasons.
In der Werkstatt ist es gefährlich, ohne Schutzbrille zu arbeiten.
In the workshop, it is dangerous to work without safety glasses.