The cigarette odor restricts the ordinary use of the apartment

Hello. I have lived in a multi-unit converted house for nearly a year, with all units owned by the same landlord. I have twice reported a strong, persistent cigarette odor entering my unit. I provided detailed logs documenting the dates, times, and duration of the smell, alongside physical symptoms I am experiencing (sore throat, headache, and burning eyes). Today, the smell lasted for hours despite closing all windows. When I requested an inspection, the landlord claimed that because smoke enters through a window, it could originate from anywhere nearby (the building, neighboring properties, or the street). He stated that without me identifying a specific location, it is too difficult to investigate, adding: "I hope you understand the difficulty in identifying the source." As a tenant, I have promptly documented and reported the hazard. I do not believe it is my responsibility to trace the origin of outdoor smoke. I am seeking legal clarity on the following questions: 1. Do tenants have a legal duty to find the exact location or structural source of incoming smoke? 2. What are the legal responsibilities of a landlord to investigate and mitigate secondhand smoke in a multi-unit property they fully own? 3. Does the practical difficulty of tracing a smell or the landlord's claim that it may originate outside legally relieve them of their duty to investigate? 4. What are my legal rights if the landlord refuses to take action? Thanks for your time and assistance.

The tenant’s statutory obligation is to notify the landlord of the defect without delay, not to identify the exact source of the cigarette odor or a structural flaw. 
The landlord must maintain the apartment in a condition suitable for its intended use throughout the term of the lease. This includes responsibility for the condition of premises under their control, as well as the building's technical systems and structural elements. If smoke enters the apartment—for instance, through the ventilation system, service penetrations, drafty windows, or due to smoking within the building—the landlord has grounds and an obligation to investigate the situation and implement measures if necessary.
The tenant is not required to prove exactly which apartment, outdoor area, or structural element is the source of the odor. However, the tenant must describe the problem in sufficient detail for the landlord to investigate it: when the odor occurs, how long it lasts, in which rooms it is detected, and whether windows and ventilation were closed or open. The logs you have kept are suitable for this purpose.
A mere assertion that the odor might originate from outside the property does not automatically absolve the landlord of the obligation to respond. Such a claim may subsequently influence which measures can actually be implemented and whether the defect falls within the landlord's area of ​​responsibility. However, following repeated specific reports, it is not justifiable to simply cite the difficulty of identifying the source without at least investigating potential causes related to the building's interior and structure.
If the smoke originates from another apartment in the same landlord's building, the obligations of that tenant must also be assessed. A residential tenant is required to consider the interests of other residents and neighbors. The landlord may be able to issue a warning to the disruptive tenant or take other contractual measures, but this requires the facts to be sufficiently established.
It is advisable to submit another written notice requesting that an inspection be arranged within a specific, reasonable timeframe and asking for a written explanation of the steps the landlord plans to take. Attach the logs kept so far and retain all correspondence. Do not reduce the rent or withhold payment on your own initiative without first considering the consequences.