Not only sometimes, always:
COVID 0.1um (microns) size
Source of this image: https://www.quora.com/What-is-the-size-of-the-Covid-19-virus?share=1 notice the propaganda on the mask filter size this is absolutely not true. You would need a full body system or better to get anywhere near 0.007um I do particulate filtering in my work as an engineer it is not possible to get this low of a particle size without spending a lot of money.
N95 mask at best will filter to 0.3um (microns) https://www.honeywell.com/en-us/newsroom/news/2020/03/n95-masks-explained This is the manufacturer’s web site for N95 masks. This means N95 or lower type masks (paper or homemade) will absolutely not stop any virus. (most viruses are this size or smaller).
Your homemade mask will never do as good as the N95 fabric lets in up to >0.1mm particles this is 1000x the size of the virus….
Once a mask is used one day it must be thoroughly washed and disinfected to be able to do anything the next day. Otherwise you are just creating a bacteria rag that you wear on your face. N95 masks are meant to be disposable (one use only) for a reason…

A summary I found with actual studies referenced below:
FACE MASKS ARE USELESS AND ENDANGER HEALTH !
In my country (Estonia), doctors spoke out and said that these rules have no scientific proof. Perhaps this was the reason why this idiotic/harmful rule was never implemented. Guess what, we are doing just fine! In Brazil for example, according to a government decree, allegedly to protect against coronavirus, all citizens must wear a face a masks in public places. People have generally followed these rules without thinking, they have blindly assumed that “experts” have given them a valid advice. However, leading medical doctors now warn that
not only do face masks fail to protect the healthy from getting sick, but they also create serious health risks to the wearer.

1. There is NO scientific support for the use of face masks!
   Careful examination of the literature, in which 17 of the best studies were analyzed, concluded that, that there is no scientific evidence necessitating the wearing of a face mask for prevention (1). Any recommendations have to be based on studies of virus transmission, however, NO studies have been done to demonstrate that either a cloth mask or the N95 mask has any effect on transmission of the COVID-19 virus. The recommendations / requirements to wear a mask are not based on any studies of this virus and have never been used to contain any other virus pandemic or epidemic in history. Therefore, government rule to ENFORCE wearing a mask has no scientific basis and is outraight dangerous to humah health.

2. Wearing a face mask endangers your health!
   Several studies have found significant problems with wearing such a mask. This can vary from headaches, to increased airway resistance, carbon dioxide accumulation, to hypoxia, all the way to serious life-threatening complications.

Masks, especially N95 respirator masks, impair respiratory exchange and are often associated with headaches. 2, 3. While straps and pressure from the mask could be causative of headaches, the bulk of the evidence points toward hypoxia and/or hypercapnia as the cause. That is, a reduction in blood oxygenation (hypoxia) or an elevation in blood C02 (hypercapnia). It is known that the N95 mask, if worn for hours, can reduce blood oxygenation as much as 20%, which can lead to a loss of consciousness and cause elderly individuals or any person with poor lung function passing out and falling, hitting their head. This, of course, can lead to death.
Researchers have found that the mask reduced the blood oxygen levels (paO2) significantly. 4. The longer the duration of wearing the mask, the greater the fall in blood oxygen levels. Studies have shown is that a drop in oxygen levels (hypoxia) is associated with an impairment in immunity. This sets the stage for contracting any infection, including COVID-19 and making the consequences of that infection much graver. In essence, your mask may put you at an increased risk of infections and if so, having a much worse outcome.5,6,7
Elderly and those with lung diseases, such as COPD, emphysema or pulmonary fibrosis, wearing a facial mask of any kind—which can cause a severe worsening of lung function. This also includes lung cancer patients and people having had lung surgery, especially with partial resection or even the removal of a whole lung.
People with cancer, especially if the cancer has spread, will be at a further risk from prolonged hypoxia as the cancer grows best in a microenvironment that is low in oxygen.

Low oxygen also promotes inflammation which can promote the growth, invasion and spread of cancers.8,9.

Repeated episodes of hypoxia has been proposed as a significant factor in atherosclerosis and hence increases all cardiovascular (heart attacks) and cerebrovascular (strokes) diseases.10
Wearing these masks on a daily basis, especially if worn for several hours is dangerous for infected people as well: They will expel some of the virus with each breath and If they are wearing a mask, they will be constantly rebreathing the viruses, raising the concentration of the virus in the lungs and the nasal passages. We know that people who have the worst reactions to the coronavirus have the highest concentrations of the virus early on. And this leads to the deadly cytokine storm in a selected number. It gets even more frightening. Evidence suggests that in some cases the virus can enter the brain(11,12). In most instances it enters the brain by way of the olfactory nerves (smell nerves), which connect directly with the area of the brain dealing with recent memory and memory consolidation.

By wearing a mask, the exhaled viruses will not be able to escape and will concentrate in the nasal passages, enter the olfactory nerves and travel into the brain.13
Wearing a mask and “social distancing” may also have serious psychological consequences for human beings and for whole human society.
The fact that this virus is a relatively benign infection for the vast majority of the population and that most of the at-risk group also survive, from an infectious disease and epidemiological standpoint, by letting the virus spread through the healthier population we will reach a herd immunity level rather quickly that will end this pandemic quickly and prevent a return. During this time, we need to protect the at-risk population by avoiding close contact, boosting their immunity with compounds that boost cellular immunity and in general, care for them.
The bottom line is that you should not wear a face mask! Government policy makers have implement a rule that has no scientific basis and is in fact harmful to citizens! This dangerous practice should be stopped immediately!

References

1. bin-Reza F et al. The use of mask and respirators to prevent transmission of influenza: A systematic review of the scientific evidence. Resp Viruses 2012;6(4):257-67. http://www.dh.gov.uk/prod_consum_dh/groups/dh_digitalassets/documents/digitalasset/dh_125425.pdf
2. Zhu JH et al. Effects of long-duration wearing of N95 respirator and surgical facemask: a pilot study. J Lung Pulm Resp Res 2014:4:97-100.
   https://medcraveonline.com/JLPRR/JLPRR-01-00021.pdf
3. Ong JJY et al. Headaches associated with personal protective equipment- A cross-sectional study among frontline healthcare workers during COVID-19. Headache 2020;60(5):864-877.
   https://headachejournal.onlinelibrary.wiley.com/doi/epdf/10.1111/head.13811
4. Bader A et al. Preliminary report on surgical mask induced deoxygenation during major surgery. Neurocirugia 2008;19:12-126.
   http://scielo.isciii.es/pdf/neuro/v19n2/3.pdf
5. Shehade H et al. Cutting edge: Hypoxia-Inducible Factor-1 negatively regulates Th1 function. J Immunol 2015;195:1372-1376.
   https://www.researchgate.net/publication/275019757_Cutting_Edge_Hypoxia-Inducible_Factor_1_Negatively_Regulates_Th1_Function
6. Westendorf AM et al. Hypoxia enhances immunosuppression by inhibiting CD4+ effector T cell function and promoting Treg activity. Cell Physiol Biochem 2017;41:1271-84.
   https://core.ac.uk/download/pdf/94541906.pdf
7. Sceneay J et al. Hypoxia-driven immunosuppression contributes to the pre-metastatic niche. Oncoimmunology 2013;2:1 e22355.
   https://ratical.org/PandemicParallaxView/onci-2-e22355.pdf
8. Blaylock RL. Immunoexcitatory mechanisms in glioma proliferation, invasion and occasional metastasis. Surg Neurol Inter 2013;4:15.
   https://www.talkingaboutthescience.com/studies/Blaylock2012.pdf
9. Aggarwal BB. Nucler factor-kappaB: The enemy within. Cancer Cell 2004;6:203-208.
   https://therayfield.com/face-masks-pose-serious-health-risks - could not find exact reference but this one is related by another MD.
10. Savransky V et al. Chronic intermittent hypoxia induces atherosclerosis. Am J Resp Crit Care Med 2007;175:1290-1297.
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3778753/
11. Baig AM et al. Evidence of the COVID-19 virus targeting the CNS: Tissue distribution, host-virus interaction, and proposed neurotropic mechanisms. ACS Chem Neurosci 2020;11:7:995-998.
    https://www.researchgate.net/publication/339916039_Evidence_of_the_COVID-19_Virus_Targeting_the_CNS_Tissue_Distribution_Host-Virus_Interaction_and_Proposed_Neurotropic_Mechanisms
12. Wu Y et al. Nervous system involvement after infection with COVID-19 and other coronaviruses. Brain Behavior, and Immunity, In press.
    https://www.sciencedirect.com/science/article/pii/S0889159120303573
13. Perlman S et al. Spread of a neurotropic murine coronavirus into the CNS via the trigeminal and olfactory nerves. Virology 1989;170:556-560.
    https://www.sciencedirect.com/science/article/pii/0042682289904467